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        <div id="contentHeaderCentre">-- Perl 5.10.0 documentation --</div>
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      <div id="breadCrumbs"><a href="index.html">Home</a> &gt; <a href="index-faq.html">FAQs</a> &gt; perlfaq7</div>
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      <div id="contentBody"><div class="title_container"><div class="page_title">perlfaq7</div></div><ul><li><a href="#NAME">NAME</a><li><a href="#DESCRIPTION">DESCRIPTION</a><ul><li><a href="#Can-I-get-a-BNF%2fyacc%2fRE-for-the-Perl-language%3f">Can I get a BNF/yacc/RE for the Perl language?</a><li><a href="#What-are-all-these-%24%40%25%26*-punctuation-signs%2c-and-how-do-I-know-when-to-use-them%3f">What are all these $@%&amp;* punctuation signs, and how do I know when to use them?</a><li><a href="#Do-I-always%2fnever-have-to-quote-my-strings-or-use-semicolons-and-commas%3f">Do I always/never have to quote my strings or use semicolons and commas?</a><li><a href="#How-do-I-skip-some-return-values%3f">How do I skip some return values?</a><li><a href="#How-do-I-temporarily-block-warnings%3f">How do I temporarily block warnings?</a><li><a href="#What's-an-extension%3f">What's an extension?</a><li><a href="#Why-do-Perl-operators-have-different-precedence-than-C-operators%3f">Why do Perl operators have different precedence than C operators?</a><li><a href="#How-do-I-declare%2fcreate-a-structure%3f">How do I declare/create a structure?</a><li><a href="#How-do-I-create-a-module%3f">How do I create a module?</a><li><a href="#How-do-I-adopt-or-take-over-a-module-already-on-CPAN%3f">How do I adopt or take over a module already on CPAN?</a><li><a href="#How-do-I-create-a-class%3f">How do I create a class?</a><li><a href="#How-can-I-tell-if-a-variable-is-tainted%3f">How can I tell if a variable is tainted?</a><li><a href="#What's-a-closure%3f">What's a closure?</a><li><a href="#What-is-variable-suicide-and-how-can-I-prevent-it%3f">What is variable suicide and how can I prevent it?</a><li><a href="#How-can-I-pass%2freturn-a-%7bFunction%2c-FileHandle%2c-Array%2c-Hash%2c-Method%2c-Regex%7d%3f">How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?</a><li><a href="#How-do-I-create-a-static-variable%3f">How do I create a static variable?</a><li><a href="#What's-the-difference-between-dynamic-and-lexical-(static)-scoping%3f--Between-local()-and-my()%3f">What's the difference between dynamic and lexical (static) scoping?  Between local() and my()?</a><li><a href="#How-can-I-access-a-dynamic-variable-while-a-similarly-named-lexical-is-in-scope%3f">How can I access a dynamic variable while a similarly named lexical is in scope?</a><li><a href="#What's-the-difference-between-deep-and-shallow-binding%3f">What's the difference between deep and shallow binding?</a><li><a href="#Why-doesn't-%22my(%24foo)-%3d-%3cFILE%3e%3b%22-work-right%3f">Why doesn't "my($foo) = &lt;FILE&gt;;" work right?</a><li><a href="#How-do-I-redefine-a-builtin-function%2c-operator%2c-or-method%3f">How do I redefine a builtin function, operator, or method?</a><li><a href="#What's-the-difference-between-calling-a-function-as-%26foo-and-foo()%3f">What's the difference between calling a function as &amp;foo and foo()?</a><li><a href="#How-do-I-create-a-switch-or-case-statement%3f">How do I create a switch or case statement?</a><li><a href="#How-can-I-catch-accesses-to-undefined-variables%2c-functions%2c-or-methods%3f">How can I catch accesses to undefined variables, functions, or methods?</a><li><a href="#Why-can't-a-method-included-in-this-same-file-be-found%3f">Why can't a method included in this same file be found?</a><li><a href="#How-can-I-find-out-my-current-package%3f">How can I find out my current package?</a><li><a href="#How-can-I-comment-out-a-large-block-of-perl-code%3f">How can I comment out a large block of perl code?</a><li><a href="#How-do-I-clear-a-package%3f">How do I clear a package?</a><li><a href="#How-can-I-use-a-variable-as-a-variable-name%3f">How can I use a variable as a variable name?</a><li><a href="#What-does-%22bad-interpreter%22-mean%3f">What does "bad interpreter" mean?</a></ul><li><a href="#REVISION">REVISION</a><li><a href="#AUTHOR-AND-COPYRIGHT">AUTHOR AND COPYRIGHT</a></ul><a name="NAME"></a><h1>NAME</h1>
<p>perlfaq7 - General Perl Language Issues ($Revision: 10100 $)</p>
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<a name="DESCRIPTION"></a><h1>DESCRIPTION</h1>
<p>This section deals with general Perl language issues that don't
clearly fit into any of the other sections.</p>
<a name="Can-I-get-a-BNF%2fyacc%2fRE-for-the-Perl-language%3f"></a><h2>Can I get a BNF/yacc/RE for the Perl language?</h2>
<p>There is no BNF, but you can paw your way through the yacc grammar in
perly.y in the source distribution if you're particularly brave.  The
grammar relies on very smart tokenizing code, so be prepared to
venture into toke.c as well.</p>
<p>In the words of Chaim Frenkel: "Perl's grammar can not be reduced to BNF.
The work of parsing perl is distributed between yacc, the lexer, smoke
and mirrors."</p>
<a name="What-are-all-these-%24%40%25%26*-punctuation-signs%2c-and-how-do-I-know-when-to-use-them%3f"></a><h2>What are all these $@%&amp;* punctuation signs, and how do I know when to use them?</h2>
<p>They are type specifiers, as detailed in <a href="perldata.html">perldata</a>:</p>
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<pre class="verbatim">	$ for scalar values (number, string or reference)
	@ for arrays
	% for hashes (associative arrays)
	&amp; for subroutines (aka functions, procedures, methods)
	* for all types of that symbol name.  In version 4 you used them like
	  pointers, but in modern perls you can just use references.</pre><p>There are couple of other symbols that you're likely to encounter that aren't
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really type specifiers:</p>
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<pre class="verbatim">	&lt;&gt; are used for inputting a record from a filehandle.
	\  takes a reference to something.</pre><p>Note that &lt;FILE&gt; is <i>neither</i> the type specifier for files
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nor the name of the handle.  It is the <code class="inline">&lt;&gt;</code>
 operator applied
to the handle FILE.  It reads one line (well, record--see
<a href="perlvar.html#%24%2f">"$/" in perlvar</a>) from the handle FILE in scalar context, or <i>all</i> lines
in list context.  When performing open, close, or any other operation
besides <code class="inline">&lt;&gt;</code>
 on files, or even when talking about the handle, do
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<i>not</i> use the brackets.  These are correct: <code class="inline"><a class="l_k" href="functions/eof.html">eof(FH)</a></code>, <code class="inline"><a class="l_k" href="functions/seek.html">seek</a><span class="s">(</span><span class="w">FH</span><span class="cm">,</span> <span class="n">0</span><span class="cm">,</span>
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<span class="n">2</span><span class="s">)</span></code>
 and "copying from STDIN to FILE".</p>
<a name="Do-I-always%2fnever-have-to-quote-my-strings-or-use-semicolons-and-commas%3f"></a><h2>Do I always/never have to quote my strings or use semicolons and commas?</h2>
<p>Normally, a bareword doesn't need to be quoted, but in most cases
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probably should be (and must be under <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">strict</span></code>
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).  But a hash key
consisting of a simple word (that isn't the name of a defined
subroutine) and the left-hand operand to the <code class="inline"><span class="cm">=&gt;</span></code>
 operator both
count as though they were quoted:</p>
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<pre class="verbatim">	This                    is like this
	------------            ---------------
	$foo{line}              $foo{'line'}
	bar =&gt; stuff            'bar' =&gt; stuff</pre><p>The final semicolon in a block is optional, as is the final comma in a
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list.  Good style (see <a href="perlstyle.html">perlstyle</a>) says to put them in except for
one-liners:</p>
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<pre class="verbatim">	if <span class="s">(</span><span class="i">$whoops</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/exit.html">exit</a> <span class="n">1</span> <span class="s">}</span>
	<span class="i">@nums</span> = <span class="s">(</span><span class="n">1</span><span class="cm">,</span> <span class="n">2</span><span class="cm">,</span> <span class="n">3</span><span class="s">)</span><span class="sc">;</span></pre>
<pre class="verbatim">	if <span class="s">(</span><span class="i">$whoops</span><span class="s">)</span> <span class="s">{</span>
		<a class="l_k" href="functions/exit.html">exit</a> <span class="n">1</span><span class="sc">;</span>
	<span class="s">}</span></pre>
<pre class="verbatim">	<span class="i">@lines</span> = <span class="s">(</span>
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	<span class="q">&quot;There Beren came from mountains cold&quot;</span><span class="cm">,</span>
	<span class="q">&quot;And lost he wandered under leaves&quot;</span><span class="cm">,</span>
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	<span class="s">)</span><span class="sc">;</span></pre>
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<a name="How-do-I-skip-some-return-values%3f"></a><h2>How do I skip some return values?</h2>
<p>One way is to treat the return values as a list and index into it:</p>
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<pre class="verbatim">	<span class="i">$dir</span> = <span class="s">(</span><a class="l_k" href="functions/getpwnam.html">getpwnam</a><span class="s">(</span><span class="i">$user</span><span class="s">)</span><span class="s">)</span>[<span class="n">7</span>]<span class="sc">;</span></pre>
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<p>Another way is to use undef as an element on the left-hand-side:</p>
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<pre class="verbatim">	<span class="s">(</span><span class="i">$dev</span><span class="cm">,</span> <span class="i">$ino</span><span class="cm">,</span> <a class="l_k" href="functions/undef.html">undef</a><span class="cm">,</span> <a class="l_k" href="functions/undef.html">undef</a><span class="cm">,</span> <span class="i">$uid</span><span class="cm">,</span> <span class="i">$gid</span><span class="s">)</span> = <a class="l_k" href="functions/stat.html">stat</a><span class="s">(</span><span class="i">$file</span><span class="s">)</span><span class="sc">;</span></pre>
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<p>You can also use a list slice to select only the elements that
you need:</p>
<pre class="verbatim">	<span class="s">(</span><span class="i">$dev</span><span class="cm">,</span> <span class="i">$ino</span><span class="cm">,</span> <span class="i">$uid</span><span class="cm">,</span> <span class="i">$gid</span><span class="s">)</span> = <span class="s">(</span> <a class="l_k" href="functions/stat.html">stat</a><span class="s">(</span><span class="i">$file</span><span class="s">)</span> <span class="s">)</span>[<span class="n">0</span><span class="cm">,</span><span class="n">1</span><span class="cm">,</span><span class="n">4</span><span class="cm">,</span><span class="n">5</span>]<span class="sc">;</span></pre>
<a name="How-do-I-temporarily-block-warnings%3f"></a><h2>How do I temporarily block warnings?</h2>
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<p>If you are running Perl 5.6.0 or better, the <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">warnings</span></code>
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 pragma
allows fine control of what warning are produced.
See <a href="perllexwarn.html">perllexwarn</a> for more details.</p>
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<pre class="verbatim">	<span class="s">{</span>
	<a class="l_k" href="functions/no.html">no</a> <span class="w">warnings</span><span class="sc">;</span>          <span class="c"># temporarily turn off warnings</span>
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	<span class="i">$a</span> = <span class="i">$b</span> + <span class="i">$c</span><span class="sc">;</span>         <span class="c"># I know these might be undef</span>
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	<span class="s">}</span></pre>
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<p>Additionally, you can enable and disable categories of warnings.
You turn off the categories you want to ignore and you can still
get other categories of warnings.  See <a href="perllexwarn.html">perllexwarn</a> for the
complete details, including the category names and hierarchy.</p>
<pre class="verbatim">	<span class="s">{</span>
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	<a class="l_k" href="functions/no.html">no</a> <span class="w">warnings</span> <span class="q">&#39;uninitialized&#39;</span><span class="sc">;</span>
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	<span class="i">$a</span> = <span class="i">$b</span> + <span class="i">$c</span><span class="sc">;</span>
	<span class="s">}</span></pre>
<p>If you have an older version of Perl, the <code class="inline"><span class="i">$^W</span></code>
 variable (documented
in <a href="perlvar.html">perlvar</a>) controls runtime warnings for a block:</p>
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<pre class="verbatim">	<span class="s">{</span>
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	<a class="l_k" href="functions/local.html">local</a> <span class="i">$^W</span> = <span class="n">0</span><span class="sc">;</span>        <span class="c"># temporarily turn off warnings</span>
	<span class="i">$a</span> = <span class="i">$b</span> + <span class="i">$c</span><span class="sc">;</span>         <span class="c"># I know these might be undef</span>
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	<span class="s">}</span></pre>
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<p>Note that like all the punctuation variables, you cannot currently
use my() on <code class="inline"><span class="i">$^W</span></code>
, only local().</p>
<a name="What's-an-extension%3f"></a><h2>What's an extension?</h2>
<p>An extension is a way of calling compiled C code from Perl.  Reading
<a href="perlxstut.html">perlxstut</a> is a good place to learn more about extensions.</p>
<a name="Why-do-Perl-operators-have-different-precedence-than-C-operators%3f"></a><h2>Why do Perl operators have different precedence than C operators?</h2>
<p>Actually, they don't.  All C operators that Perl copies have the same
precedence in Perl as they do in C.  The problem is with operators that C
doesn't have, especially functions that give a list context to everything
on their right, eg. print, chmod, exec, and so on.  Such functions are
called "list operators" and appear as such in the precedence table in
<a href="perlop.html">perlop</a>.</p>
<p>A common mistake is to write:</p>
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<pre class="verbatim">	<a class="l_k" href="functions/unlink.html">unlink</a> <span class="i">$file</span> || <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;snafu&quot;</span><span class="sc">;</span></pre>
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<p>This gets interpreted as:</p>
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<pre class="verbatim">	<a class="l_k" href="functions/unlink.html">unlink</a> <span class="s">(</span><span class="i">$file</span> || <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;snafu&quot;</span><span class="s">)</span><span class="sc">;</span></pre>
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<p>To avoid this problem, either put in extra parentheses or use the
super low precedence <code class="inline">or</code>
 operator:</p>
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<pre class="verbatim">	<span class="s">(</span><a class="l_k" href="functions/unlink.html">unlink</a> <span class="i">$file</span><span class="s">)</span> || <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;snafu&quot;</span><span class="sc">;</span>
	<a class="l_k" href="functions/unlink.html">unlink</a> <span class="i">$file</span> or <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;snafu&quot;</span><span class="sc">;</span></pre>
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<p>The "English" operators (<code class="inline">and</code>
, <code class="inline">or</code>
, <code class="inline">xor</code>
, and <code class="inline">not</code>
)
deliberately have precedence lower than that of list operators for
just such situations as the one above.</p>
<p>Another operator with surprising precedence is exponentiation.  It
binds more tightly even than unary minus, making <code class="inline"><span class="n">-2</span>**<span class="n">2</span></code>
 product a
negative not a positive four.  It is also right-associating, meaning
that <code class="inline"><span class="n">2</span>**<span class="n">3</span>**<span class="n">2</span></code>
 is two raised to the ninth power, not eight squared.</p>
<p>Although it has the same precedence as in C, Perl's <code class="inline">?:</code> operator
produces an lvalue.  This assigns $x to either $a or $b, depending
on the trueness of $maybe:</p>
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<pre class="verbatim">	<span class="s">(</span><span class="i">$maybe</span> ? <span class="i">$a</span> <span class="co">:</span> <span class="i">$b</span><span class="s">)</span> = <span class="i">$x</span><span class="sc">;</span></pre>
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<a name="How-do-I-declare%2fcreate-a-structure%3f"></a><h2>How do I declare/create a structure?</h2>
<p>In general, you don't "declare" a structure.  Just use a (probably
anonymous) hash reference.  See <a href="perlref.html">perlref</a> and <a href="perldsc.html">perldsc</a> for details.
Here's an example:</p>
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<pre class="verbatim">	<span class="i">$person</span> = <span class="s">{</span><span class="s">}</span><span class="sc">;</span>                   <span class="c"># new anonymous hash</span>
	<span class="i">$person</span>-&gt;{<span class="w">AGE</span>}  = <span class="n">24</span><span class="sc">;</span>           <span class="c"># set field AGE to 24</span>
	<span class="i">$person</span>-&gt;{<span class="w">NAME</span>} = <span class="q">&quot;Nat&quot;</span><span class="sc">;</span>        <span class="c"># set field NAME to &quot;Nat&quot;</span></pre>
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<p>If you're looking for something a bit more rigorous, try <a href="perltoot.html">perltoot</a>.</p>
<a name="How-do-I-create-a-module%3f"></a><h2>How do I create a module?</h2>
<p>(contributed by brian d foy)</p>
<p><a href="perlmod.html">perlmod</a>, <a href="perlmodlib.html">perlmodlib</a>, <a href="perlmodstyle.html">perlmodstyle</a> explain modules
in all the gory details. <a href="perlnewmod.html">perlnewmod</a> gives a brief
overview of the process along with a couple of suggestions
about style.</p>
<p>If you need to include C code or C library interfaces in
your module, you'll need h2xs.  h2xs will create the module
distribution structure and the initial interface files
you'll need.  <a href="perlxs.html">perlxs</a> and <a href="perlxstut.html">perlxstut</a> explain the details.</p>
<p>If you don't need to use C code, other tools such as
ExtUtils::ModuleMaker and Module::Starter, can help you
create a skeleton module distribution.</p>
<p>You may also want to see Sam Tregar's "Writing Perl Modules
for CPAN" ( <a href="http://apress.com/book/bookDisplay.html?bID=14">http://apress.com/book/bookDisplay.html?bID=14</a> )
which is the best hands-on guide to creating module
distributions.</p>
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<a name="How-do-I-adopt-or-take-over-a-module-already-on-CPAN%3f"></a><h2>How do I adopt or take over a module already on CPAN?</h2>
<p>(contributed by brian d foy)</p>
<p>The easiest way to take over a module is to have the current
module maintainer either make you a co-maintainer or transfer
the module to you.</p>
<p>If you can't reach the author for some reason (e.g. email bounces),
the PAUSE admins at modules@perl.org can help. The PAUSE admins
treat each case individually.</p>
<ul>
<li>
<p>Get a login for the Perl Authors Upload Server (PAUSE) if you don't
already have one: <a href="http://pause.perl.org">http://pause.perl.org</a></p>
</li>
<li>
<p>Write to modules@perl.org explaining what you did to contact the
current maintainer. The PAUSE admins will also try to reach the
maintainer.</p>
</li>
<li>
<p>Post a public message in a heavily trafficked site announcing your
intention to take over the module.</p>
</li>
<li>
<p>Wait a bit. The PAUSE admins don't want to act too quickly in case
the current maintainer is on holiday. If there's no response to 
private communication or the public post, a PAUSE admin can transfer
it to you.</p>
</li>
</ul>
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<a name="How-do-I-create-a-class%3f"></a><h2>How do I create a class?</h2>
<p>See <a href="perltoot.html">perltoot</a> for an introduction to classes and objects, as well as
<a href="perlobj.html">perlobj</a> and <a href="perlbot.html">perlbot</a>.</p>
<a name="How-can-I-tell-if-a-variable-is-tainted%3f"></a><h2>How can I tell if a variable is tainted?</h2>
<p>You can use the tainted() function of the Scalar::Util module, available
from CPAN (or included with Perl since release 5.8.0).
See also <a href="perlsec.html#Laundering-and-Detecting-Tainted-Data">"Laundering and Detecting Tainted Data" in perlsec</a>.</p>
<a name="What's-a-closure%3f"></a><h2>What's a closure?</h2>
<p>Closures are documented in <a href="perlref.html">perlref</a>.</p>
<p><i>Closure</i> is a computer science term with a precise but
279 280 281 282 283 284 285 286 287
hard-to-explain meaning. Usually, closures are implemented in Perl as
anonymous subroutines with lasting references to lexical variables
outside their own scopes. These lexicals magically refer to the
variables that were around when the subroutine was defined (deep 
binding).</p>
<p>Closures are most often used in programming languages where you can
have the return value of a function be itself a function, as you can
in Perl. Note that some languages provide anonymous functions but are
not capable of providing proper closures: the Python language, for
288 289 290
example.  For more information on closures, check out any textbook on
functional programming.  Scheme is a language that not only supports
but encourages closures.</p>
291 292 293 294 295 296 297 298 299 300
<p>Here's a classic non-closure function-generating function:</p>
<pre class="verbatim"><a name="add_function_generator"></a>	sub <span class="m">add_function_generator</span> <span class="s">{</span>
		<a class="l_k" href="functions/return.html">return</a> <a class="l_k" href="functions/sub.html">sub</a> <span class="s">{</span> <a class="l_k" href="functions/shift.html">shift</a><span class="s">(</span><span class="s">)</span> + <a class="l_k" href="functions/shift.html">shift</a><span class="s">(</span><span class="s">)</span> <span class="s">}</span><span class="sc">;</span>
		<span class="s">}</span></pre>
<pre class="verbatim">	<span class="i">$add_sub</span> = <span class="i">add_function_generator</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span>
	<span class="i">$sum</span> = <span class="i">$add_sub</span>-&gt;<span class="s">(</span><span class="n">4</span><span class="cm">,</span><span class="n">5</span><span class="s">)</span><span class="sc">;</span>                <span class="c"># $sum is 9 now.</span></pre>
<p>The anonymous subroutine returned by add_function_generator() isn't
technically a closure because it refers to no lexicals outside its own
scope.  Using a closure gives you a <i>function template</i> with some
customization slots left out to be filled later.</p>
301 302 303 304 305
<p>Contrast this with the following make_adder() function, in which the
returned anonymous function contains a reference to a lexical variable
outside the scope of that function itself.  Such a reference requires
that Perl return a proper closure, thus locking in for all time the
value that the lexical had when the function was created.</p>
306 307 308 309 310 311
<pre class="verbatim"><a name="make_adder"></a>	sub <span class="m">make_adder</span> <span class="s">{</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$addpiece</span> = <a class="l_k" href="functions/shift.html">shift</a><span class="sc">;</span>
		<a class="l_k" href="functions/return.html">return</a> <a class="l_k" href="functions/sub.html">sub</a> <span class="s">{</span> <a class="l_k" href="functions/shift.html">shift</a><span class="s">(</span><span class="s">)</span> + <span class="i">$addpiece</span> <span class="s">}</span><span class="sc">;</span>
	<span class="s">}</span></pre>
<pre class="verbatim">	<span class="i">$f1</span> = <span class="i">make_adder</span><span class="s">(</span><span class="n">20</span><span class="s">)</span><span class="sc">;</span>
	<span class="i">$f2</span> = <span class="i">make_adder</span><span class="s">(</span><span class="n">555</span><span class="s">)</span><span class="sc">;</span></pre>
312 313 314 315 316 317 318
<p>Now <code class="inline"><span class="i">&amp;$f1</span><span class="s">(</span><span class="i">$n</span><span class="s">)</span></code>
 is always 20 plus whatever $n you pass in, whereas
<code class="inline"><span class="i">&amp;$f2</span><span class="s">(</span><span class="i">$n</span><span class="s">)</span></code>
 is always 555 plus whatever $n you pass in.  The $addpiece
in the closure sticks around.</p>
<p>Closures are often used for less esoteric purposes.  For example, when
you want to pass in a bit of code into a function:</p>
319 320
<pre class="verbatim">	<a class="l_k" href="functions/my.html">my</a> <span class="i">$line</span><span class="sc">;</span>
	<span class="i">timeout</span><span class="s">(</span> <span class="n">30</span><span class="cm">,</span> <a class="l_k" href="functions/sub.html">sub</a> <span class="s">{</span> <span class="i">$line</span> = <span class="q">&lt;STDIN&gt;</span> <span class="s">}</span> <span class="s">)</span><span class="sc">;</span></pre>
321 322 323 324 325
<p>If the code to execute had been passed in as a string,
<code class="inline"><span class="q">&#39;$line = &lt;STDIN&gt;&#39;</span></code>
, there would have been no way for the
hypothetical timeout() function to access the lexical variable
$line back in its caller's scope.</p>
326 327 328 329 330 331 332 333 334 335 336 337
<p>Another use for a closure is to make a variable <i>private</i> to a
named subroutine, e.g. a counter that gets initialized at creation
time of the sub and can only be modified from within the sub.
This is sometimes used with a BEGIN block in package files to make
sure a variable doesn't get meddled with during the lifetime of the
package:</p>
<pre class="verbatim">	BEGIN <span class="s">{</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$id</span> = <span class="n">0</span><span class="sc">;</span>
<a name="next_id"></a>		sub <span class="m">next_id</span> <span class="s">{</span> ++<span class="i">$id</span> <span class="s">}</span>
	<span class="s">}</span></pre>
<p>This is discussed in more detail in <a href="perlsub.html">perlsub</a>, see the entry on
<i>Persistent Private Variables</i>.</p>
338 339 340 341 342 343 344 345
<a name="What-is-variable-suicide-and-how-can-I-prevent-it%3f"></a><h2>What is variable suicide and how can I prevent it?</h2>
<p>This problem was fixed in perl 5.004_05, so preventing it means upgrading
your version of perl. ;)</p>
<p>Variable suicide is when you (temporarily or permanently) lose the value
of a variable.  It is caused by scoping through my() and local()
interacting with either closures or aliased foreach() iterator variables
and subroutine arguments.  It used to be easy to inadvertently lose a
variable's value this way, but now it's much harder.  Take this code:</p>
346 347 348 349 350 351
<pre class="verbatim">	<a class="l_k" href="functions/my.html">my</a> <span class="i">$f</span> = <span class="q">'foo'</span><span class="sc">;</span>
<a name="T"></a>	sub <span class="m">T</span> <span class="s">{</span>
		while <span class="s">(</span><span class="i">$i</span>++ &lt; <span class="n">3</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/my.html">my</a> <span class="i">$f</span> = <span class="i">$f</span><span class="sc">;</span> <span class="i">$f</span> .= <span class="q">&quot;bar&quot;</span><span class="sc">;</span> <a class="l_k" href="functions/print.html">print</a> <span class="i">$f</span><span class="cm">,</span> <span class="q">&quot;\n&quot;</span> <span class="s">}</span>
		<span class="s">}</span></pre>
<pre class="verbatim">	<span class="w">T</span><span class="sc">;</span>
	<a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Finally $f\n&quot;</span><span class="sc">;</span></pre>
352 353 354 355 356 357 358
<p>If you are experiencing variable suicide, that <code class="inline"><a class="l_k" href="functions/my.html">my</a> <span class="i">$f</span></code>
 in the subroutine
doesn't pick up a fresh copy of the <code class="inline"><span class="i">$f</span></code>
 whose value is &lt;foo&gt;. The output
shows that inside the subroutine the value of <code class="inline"><span class="i">$f</span></code>
 leaks through when it
shouldn't, as in this output:</p>
359 360 361 362
<pre class="verbatim">	<span class="w">foobar</span>
	<span class="w">foobarbar</span>
	<span class="w">foobarbarbar</span>
	<span class="w">Finally</span> <span class="w">foo</span></pre>
363 364 365 366 367
<p>The $f that has "bar" added to it three times should be a new <code class="inline"><span class="i">$f</span></code>

<code class="inline"><a class="l_k" href="functions/my.html">my</a> <span class="i">$f</span></code>
 should create a new lexical variable each time through the loop.
The expected output is:</p>
368 369 370 371
<pre class="verbatim">	<span class="w">foobar</span>
	<span class="w">foobar</span>
	<span class="w">foobar</span>
	<span class="w">Finally</span> <span class="w">foo</span></pre>
372 373 374 375
<a name="How-can-I-pass%2freturn-a-%7bFunction%2c-FileHandle%2c-Array%2c-Hash%2c-Method%2c-Regex%7d%3f"></a><h2>How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?</h2>
<p>With the exception of regexes, you need to pass references to these
objects.  See <a href="perlsub.html#Pass-by-Reference">"Pass by Reference" in perlsub</a> for this particular
question, and <a href="perlref.html">perlref</a> for information on references.</p>
376 377
<p>See "Passing Regexes", later in <a href="perlfaq7.html">perlfaq7</a>, for information on
passing regular expressions.</p>
378 379 380 381
<ul>
<li><a name="Passing-Variables-and-Functions"></a><b>Passing Variables and Functions</b>
<p>Regular variables and functions are quite easy to pass: just pass in a
reference to an existing or anonymous variable or function:</p>
382 383 384 385 386 387 388
<pre class="verbatim">	<span class="i">func</span><span class="s">(</span> \<span class="i">$some_scalar</span> <span class="s">)</span><span class="sc">;</span></pre>
<pre class="verbatim">	<span class="i">func</span><span class="s">(</span> \<span class="i">@some_array</span>  <span class="s">)</span><span class="sc">;</span>
	<span class="i">func</span><span class="s">(</span> <span class="s">[</span> <span class="n">1</span> .. <span class="n">10</span> <span class="s">]</span>   <span class="s">)</span><span class="sc">;</span></pre>
<pre class="verbatim">	<span class="i">func</span><span class="s">(</span> \<span class="i">%some_hash</span>   <span class="s">)</span><span class="sc">;</span>
	<span class="i">func</span><span class="s">(</span> <span class="s">{</span> <span class="w">this</span> <span class="cm">=&gt;</span> <span class="n">10</span><span class="cm">,</span> <span class="w">that</span> <span class="cm">=&gt;</span> <span class="n">20</span> <span class="s">}</span>   <span class="s">)</span><span class="sc">;</span></pre>
<pre class="verbatim">	<span class="i">func</span><span class="s">(</span> \<span class="i">&amp;some_func</span>   <span class="s">)</span><span class="sc">;</span>
	<span class="i">func</span><span class="s">(</span> <a class="l_k" href="functions/sub.html">sub</a> <span class="s">{</span> <span class="i">$_</span>[<span class="n">0</span>] ** <span class="i">$_</span>[<span class="n">1</span>] <span class="s">}</span>   <span class="s">)</span><span class="sc">;</span></pre>
389 390 391 392 393 394 395
</li>
<li><a name="Passing-Filehandles"></a><b>Passing Filehandles</b>
<p>As of Perl 5.6, you can represent filehandles with scalar variables
which you treat as any other scalar.</p>
<pre class="verbatim">	<a class="l_k" href="functions/open.html">open</a> <a class="l_k" href="functions/my.html">my</a> <span class="i">$fh</span><span class="cm">,</span> <span class="i">$filename</span> or <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;Cannot open $filename! $!&quot;</span><span class="sc">;</span>
	<span class="i">func</span><span class="s">(</span> <span class="i">$fh</span> <span class="s">)</span><span class="sc">;</span></pre>
<pre class="verbatim">	sub func {
396
		my $passed_fh = shift;</pre><pre class="verbatim">		my $line = &lt;$passed_fh&gt;;
397 398 399 400 401 402 403 404 405 406 407 408
		}</pre><p>Before Perl 5.6, you had to use the <code class="inline"><span class="i">*FH</span></code>
 or <code class="inline">\<span class="i">*FH</span></code>
 notations.
These are "typeglobs"--see <a href="perldata.html#Typeglobs-and-Filehandles">"Typeglobs and Filehandles" in perldata</a>
and especially <a href="perlsub.html#Pass-by-Reference">"Pass by Reference" in perlsub</a> for more information.</p>
</li>
<li><a name="Passing-Regexes"></a><b>Passing Regexes</b>
<p>To pass regexes around, you'll need to be using a release of Perl
sufficiently recent as to support the <code class="inline"><a class="l_k" href="functions/qr.html">qr//</a></code> construct, pass around
strings and use an exception-trapping eval, or else be very, very clever.</p>
<p>Here's an example of how to pass in a string to be regex compared
using <code class="inline"><a class="l_k" href="functions/qr.html">qr//</a></code>:</p>
409 410 411
<pre class="verbatim"><a name="compare"></a>	sub <span class="m">compare($$)</span> <span class="s">{</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="s">(</span><span class="i">$val1</span><span class="cm">,</span> <span class="i">$regex</span><span class="s">)</span> = <span class="i">@_</span><span class="sc">;</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$retval</span> = <span class="i">$val1</span> =~ <span class="q">/$regex/</span><span class="sc">;</span>
412
	<a class="l_k" href="functions/return.html">return</a> <span class="i">$retval</span><span class="sc">;</span>
413 414
	<span class="s">}</span>
	<span class="i">$match</span> = <span class="i">compare</span><span class="s">(</span><span class="q">&quot;old McDonald&quot;</span><span class="cm">,</span> <span class="q">qr/d.*D/i</span><span class="s">)</span><span class="sc">;</span></pre>
415 416 417 418 419
<p>Notice how <code class="inline"><a class="l_k" href="functions/qr.html">qr//</a></code> allows flags at the end.  That pattern was compiled
at compile time, although it was executed later.  The nifty <code class="inline"><a class="l_k" href="functions/qr.html">qr//</a></code>
notation wasn't introduced until the 5.005 release.  Before that, you
had to approach this problem much less intuitively.  For example, here
it is again if you don't have <code class="inline"><a class="l_k" href="functions/qr.html">qr//</a></code>:</p>
420 421 422
<pre class="verbatim"><a name="compare"></a>	sub <span class="m">compare($$)</span> <span class="s">{</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="s">(</span><span class="i">$val1</span><span class="cm">,</span> <span class="i">$regex</span><span class="s">)</span> = <span class="i">@_</span><span class="sc">;</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$retval</span> = <a class="l_k" href="functions/eval.html">eval</a> <span class="s">{</span> <span class="i">$val1</span> =~ <span class="q">/$regex/</span> <span class="s">}</span><span class="sc">;</span>
423 424
	<a class="l_k" href="functions/die.html">die</a> if <span class="i">$@</span><span class="sc">;</span>
	<a class="l_k" href="functions/return.html">return</a> <span class="i">$retval</span><span class="sc">;</span>
425 426
	<span class="s">}</span></pre>
<pre class="verbatim">	<span class="i">$match</span> = <span class="i">compare</span><span class="s">(</span><span class="q">&quot;old McDonald&quot;</span><span class="cm">,</span> <span class="q">q/($?i)d.*D/</span><span class="s">)</span><span class="sc">;</span></pre>
427
<p>Make sure you never say something like this:</p>
428
<pre class="verbatim">	<a class="l_k" href="functions/return.html">return</a> <a class="l_k" href="functions/eval.html">eval</a> <span class="q">&quot;\$val =~ /$regex/&quot;</span><span class="sc">;</span>   <span class="c"># WRONG</span></pre>
429 430
<p>or someone can sneak shell escapes into the regex due to the double
interpolation of the eval and the double-quoted string.  For example:</p>
431 432
<pre class="verbatim">	<span class="i">$pattern_of_evil</span> = <span class="q">'danger ${ system(&quot;rm -rf * &amp;&quot;) } danger'</span><span class="sc">;</span></pre>
<pre class="verbatim">	<a class="l_k" href="functions/eval.html">eval</a> <span class="q">&quot;\$string =~ /$pattern_of_evil/&quot;</span><span class="sc">;</span></pre>
433 434 435 436 437 438 439
<p>Those preferring to be very, very clever might see the O'Reilly book,
<i>Mastering Regular Expressions</i>, by Jeffrey Friedl.  Page 273's
Build_MatchMany_Function() is particularly interesting.  A complete
citation of this book is given in <a href="perlfaq2.html">perlfaq2</a>.</p>
</li>
<li><a name="Passing-Methods"></a><b>Passing Methods</b>
<p>To pass an object method into a subroutine, you can do this:</p>
440 441 442 443 444 445 446
<pre class="verbatim">	call_a_lot(10, $some_obj, "methname")
	sub call_a_lot {
		my ($count, $widget, $trick) = @_;
		for (my $i = 0; $i &lt; $count; $i++) {
			$widget-&gt;$trick();
		}
	}</pre><p>Or, you can use a closure to bundle up the object, its
447
method call, and arguments:</p>
448 449 450 451 452 453
<pre class="verbatim">	<a class="l_k" href="functions/my.html">my</a> <span class="i">$whatnot</span> =  <a class="l_k" href="functions/sub.html">sub</a> <span class="s">{</span> <span class="i">$some_obj</span><span class="i">-&gt;obfuscate</span><span class="s">(</span><span class="i">@args</span><span class="s">)</span> <span class="s">}</span><span class="sc">;</span>
	<span class="i">func</span><span class="s">(</span><span class="i">$whatnot</span><span class="s">)</span><span class="sc">;</span>
<a name="func"></a>	sub <span class="m">func</span> <span class="s">{</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$code</span> = <a class="l_k" href="functions/shift.html">shift</a><span class="sc">;</span>
		<span class="i">&amp;$code</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span>
	<span class="s">}</span></pre>
454 455 456 457 458 459 460 461 462 463
<p>You could also investigate the can() method in the UNIVERSAL class
(part of the standard perl distribution).</p>
</li>
</ul>
<a name="How-do-I-create-a-static-variable%3f"></a><h2>How do I create a static variable?</h2>
<p>(contributed by brian d foy)</p>
<p>Perl doesn't have "static" variables, which can only be accessed from
the function in which they are declared. You can get the same effect
with lexical variables, though.</p>
<p>You can fake a static variable by using a lexical variable which goes
464
out of scope. In this example, you define the subroutine <code class="inline"><span class="w">counter</span></code>
465 466 467 468 469 470 471 472 473 474
, and
it uses the lexical variable <code class="inline"><span class="i">$count</span></code>
. Since you wrap this in a BEGIN
block, <code class="inline"><span class="i">$count</span></code>
 is defined at compile-time, but also goes out of
scope at the end of the BEGIN block. The BEGIN block also ensures that
the subroutine and the value it uses is defined at compile-time so the
subroutine is ready to use just like any other subroutine, and you can
put this code in the same place as other subroutines in the program
text (i.e. at the end of the code, typically). The subroutine
475
<code class="inline"><span class="w">counter</span></code>
476 477 478 479
 still has a reference to the data, and is the only way you
can access the value (and each time you do, you increment the value).
The data in chunk of memory defined by <code class="inline"><span class="i">$count</span></code>
 is private to
480
<code class="inline"><span class="w">counter</span></code>
481
.</p>
482 483 484 485 486 487
<pre class="verbatim">	BEGIN <span class="s">{</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$count</span> = <span class="n">1</span><span class="sc">;</span>
<a name="counter"></a>		sub <span class="m">counter</span> <span class="s">{</span> <span class="i">$count</span>++ <span class="s">}</span>
	<span class="s">}</span></pre>
<pre class="verbatim">	<a class="l_k" href="functions/my.html">my</a> <span class="i">$start</span> = <span class="i">counter</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span></pre>
<pre class="verbatim">	.... # code that calls counter();</pre><pre class="verbatim">	<a class="l_k" href="functions/my.html">my</a> <span class="i">$end</span> = <span class="i">counter</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span></pre>
488 489 490 491 492 493
<p>In the previous example, you created a function-private variable
because only one function remembered its reference. You could define
multiple functions while the variable is in scope, and each function
can share the "private" variable. It's not really "static" because you
can access it outside the function while the lexical variable is in
scope, and even create references to it. In this example,
494 495
<code class="inline"><span class="w">increment_count</span></code>
 and <code class="inline"><span class="w">return_count</span></code>
496 497 498 499 500
 share the variable. One
function adds to the value and the other simply returns the value.
They can both access <code class="inline"><span class="i">$count</span></code>
, and since it has gone out of scope,
there is no other way to access it.</p>
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<pre class="verbatim">	BEGIN <span class="s">{</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$count</span> = <span class="n">1</span><span class="sc">;</span>
<a name="increment_count"></a>		sub <span class="m">increment_count</span> <span class="s">{</span> <span class="i">$count</span>++ <span class="s">}</span>
<a name="return_count"></a>		sub <span class="m">return_count</span>    <span class="s">{</span> <span class="i">$count</span> <span class="s">}</span>
	<span class="s">}</span></pre>
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<p>To declare a file-private variable, you still use a lexical variable.
A file is also a scope, so a lexical variable defined in the file
cannot be seen from any other file.</p>
<p>See <a href="perlsub.html#Persistent-Private-Variables">"Persistent Private Variables" in perlsub</a> for more information.
The discussion of closures in <a href="perlref.html">perlref</a> may help you even though we
did not use anonymous subroutines in this answer. See
<a href="perlsub.html#Persistent-Private-Variables">"Persistent Private Variables" in perlsub</a> for details.</p>
<a name="What's-the-difference-between-dynamic-and-lexical-(static)-scoping%3f--Between-local()-and-my()%3f"></a><h2>What's the difference between dynamic and lexical (static) scoping?  Between local() and my()?</h2>
<p><code class="inline"><a class="l_k" href="functions/local.html">local($x)</a></code> saves away the old value of the global variable <code class="inline"><span class="i">$x</span></code>

and assigns a new value for the duration of the subroutine <i>which is
visible in other functions called from that subroutine</i>.  This is done
at run-time, so is called dynamic scoping.  local() always affects global
variables, also called package variables or dynamic variables.</p>
<p><code class="inline"><a class="l_k" href="functions/my.html">my($x)</a></code> creates a new variable that is only visible in the current
subroutine.  This is done at compile-time, so it is called lexical or
static scoping.  my() always affects private variables, also called
lexical variables or (improperly) static(ly scoped) variables.</p>
<p>For instance:</p>
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<pre class="verbatim"><a name="visible"></a>	sub <span class="m">visible</span> <span class="s">{</span>
		<a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;var has value $var\n&quot;</span><span class="sc">;</span>
		<span class="s">}</span></pre>
<pre class="verbatim"><a name="dynamic"></a>	sub <span class="m">dynamic</span> <span class="s">{</span>
		<a class="l_k" href="functions/local.html">local</a> <span class="i">$var</span> = <span class="q">'local'</span><span class="sc">;</span>	<span class="c"># new temporary value for the still-global</span>
		<span class="i">visible</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span>              <span class="c">#   variable called $var</span>
		<span class="s">}</span></pre>
<pre class="verbatim"><a name="lexical"></a>	sub <span class="m">lexical</span> <span class="s">{</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$var</span> = <span class="q">'private'</span><span class="sc">;</span>    <span class="c"># new private variable, $var</span>
		<span class="i">visible</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span>              <span class="c"># (invisible outside of sub scope)</span>
		<span class="s">}</span></pre>
<pre class="verbatim">	<span class="i">$var</span> = <span class="q">'global'</span><span class="sc">;</span></pre>
<pre class="verbatim">	<span class="i">visible</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span>      		<span class="c"># prints global</span>
	<span class="i">dynamic</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span>      		<span class="c"># prints local</span>
	<span class="i">lexical</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span>      		<span class="c"># prints global</span></pre>
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<p>Notice how at no point does the value "private" get printed.  That's
because $var only has that value within the block of the lexical()
function, and it is hidden from called subroutine.</p>
<p>In summary, local() doesn't make what you think of as private, local
variables.  It gives a global variable a temporary value.  my() is
what you're looking for if you want private variables.</p>
<p>See <a href="perlsub.html#Private-Variables-via-my()">"Private Variables via my()" in perlsub</a> and
<a href="perlsub.html#Temporary-Values-via-local()">"Temporary Values via local()" in perlsub</a> for excruciating details.</p>
<a name="How-can-I-access-a-dynamic-variable-while-a-similarly-named-lexical-is-in-scope%3f"></a><h2>How can I access a dynamic variable while a similarly named lexical is in scope?</h2>
<p>If you know your package, you can just mention it explicitly, as in
$Some_Pack::var. Note that the notation $::var is <b>not</b> the dynamic $var
in the current package, but rather the one in the "main" package, as
though you had written $main::var.</p>
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<pre class="verbatim">	<a class="l_k" href="functions/use.html">use</a> <span class="w">vars</span> <span class="q">&#39;$var&#39;</span><span class="sc">;</span>
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	<a class="l_k" href="functions/local.html">local</a> <span class="i">$var</span> = <span class="q">&quot;global&quot;</span><span class="sc">;</span>
	<a class="l_k" href="functions/my.html">my</a>    <span class="i">$var</span> = <span class="q">&quot;lexical&quot;</span><span class="sc">;</span></pre>
<pre class="verbatim">	<a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;lexical is $var\n&quot;</span><span class="sc">;</span>
	<a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;global  is $main::var\n&quot;</span><span class="sc">;</span></pre>
<p>Alternatively you can use the compiler directive our() to bring a
dynamic variable into the current lexical scope.</p>
<pre class="verbatim">	<a class="l_k" href="functions/require.html">require</a> <span class="n">5.006</span><span class="sc">;</span> <span class="c"># our() did not exist before 5.6</span>
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	<a class="l_k" href="functions/use.html">use</a> <span class="w">vars</span> <span class="q">&#39;$var&#39;</span><span class="sc">;</span></pre>
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<pre class="verbatim">	<a class="l_k" href="functions/local.html">local</a> <span class="i">$var</span> = <span class="q">&quot;global&quot;</span><span class="sc">;</span>
	<a class="l_k" href="functions/my.html">my</a> <span class="i">$var</span>    = <span class="q">&quot;lexical&quot;</span><span class="sc">;</span></pre>
<pre class="verbatim">	<a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;lexical is $var\n&quot;</span><span class="sc">;</span></pre>
<pre class="verbatim">	<span class="s">{</span>
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		<a class="l_k" href="functions/our.html">our</a> <span class="i">$var</span><span class="sc">;</span>
		<a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;global  is $var\n&quot;</span><span class="sc">;</span>
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	<span class="s">}</span></pre>
<a name="What's-the-difference-between-deep-and-shallow-binding%3f"></a><h2>What's the difference between deep and shallow binding?</h2>
<p>In deep binding, lexical variables mentioned in anonymous subroutines
are the same ones that were in scope when the subroutine was created.
In shallow binding, they are whichever variables with the same names
happen to be in scope when the subroutine is called.  Perl always uses
deep binding of lexical variables (i.e., those created with my()).
However, dynamic variables (aka global, local, or package variables)
are effectively shallowly bound.  Consider this just one more reason
not to use them.  See the answer to <a href="#What's-a-closure%3f">"What's a closure?"</a>.</p>
<a name="Why-doesn't-%22my(%24foo)-%3d-%3cFILE%3e%3b%22-work-right%3f"></a><h2>Why doesn't "my($foo) = &lt;FILE&gt;;" work right?</h2>
<p><code class="inline"><a class="l_k" href="functions/my.html">my()</a></code> and <code class="inline"><a class="l_k" href="functions/local.html">local()</a></code> give list context to the right hand side
of <code class="inline">=</code>
.  The &lt;FH&gt; read operation, like so many of Perl's
functions and operators, can tell which context it was called in and
behaves appropriately.  In general, the scalar() function can help.
This function does nothing to the data itself (contrary to popular myth)
but rather tells its argument to behave in whatever its scalar fashion is.
If that function doesn't have a defined scalar behavior, this of course
doesn't help you (such as with sort()).</p>
<p>To enforce scalar context in this particular case, however, you need
merely omit the parentheses:</p>
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<pre class="verbatim">	<a class="l_k" href="functions/local.html">local</a><span class="s">(</span><span class="i">$foo</span><span class="s">)</span> = <span class="q">&lt;FILE&gt;</span><span class="sc">;</span>	    <span class="c"># WRONG</span>
	<a class="l_k" href="functions/local.html">local</a><span class="s">(</span><span class="i">$foo</span><span class="s">)</span> = <a class="l_k" href="functions/scalar.html">scalar</a><span class="s">(</span><span class="q">&lt;FILE&gt;</span><span class="s">)</span><span class="sc">;</span>   <span class="c"># ok</span>
	<a class="l_k" href="functions/local.html">local</a> <span class="i">$foo</span>  = <span class="q">&lt;FILE&gt;</span><span class="sc">;</span>	    <span class="c"># right</span></pre>
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<p>You should probably be using lexical variables anyway, although the
issue is the same here:</p>
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<pre class="verbatim">	<a class="l_k" href="functions/my.html">my</a><span class="s">(</span><span class="i">$foo</span><span class="s">)</span> = <span class="q">&lt;FILE&gt;</span><span class="sc">;</span>	<span class="c"># WRONG</span>
	<a class="l_k" href="functions/my.html">my</a> <span class="i">$foo</span>  = <span class="q">&lt;FILE&gt;</span><span class="sc">;</span>	<span class="c"># right</span></pre>
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<a name="How-do-I-redefine-a-builtin-function%2c-operator%2c-or-method%3f"></a><h2>How do I redefine a builtin function, operator, or method?</h2>
<p>Why do you want to do that? :-)</p>
<p>If you want to override a predefined function, such as open(),
then you'll have to import the new definition from a different
module.  See <a href="perlsub.html#Overriding-Built-in-Functions">"Overriding Built-in Functions" in perlsub</a>.  There's
also an example in <a href="perltoot.html#Class%3a%3aTemplate">"Class::Template" in perltoot</a>.</p>
<p>If you want to overload a Perl operator, such as <code class="inline">+</code>
 or <code class="inline"><span class="i">**</span></code>
,
606
then you'll want to use the <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">overload</span></code>
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 pragma, documented
in <a href="overload.html">overload</a>.</p>
<p>If you're talking about obscuring method calls in parent classes,
see <a href="perltoot.html#Overridden-Methods">"Overridden Methods" in perltoot</a>.</p>
<a name="What's-the-difference-between-calling-a-function-as-%26foo-and-foo()%3f"></a><h2>What's the difference between calling a function as &amp;foo and foo()?</h2>
<p>When you call a function as <code class="inline"><span class="i">&amp;foo</span></code>
, you allow that function access to
your current @_ values, and you bypass prototypes.
The function doesn't get an empty @_--it gets yours!  While not
strictly speaking a bug (it's documented that way in <a href="perlsub.html">perlsub</a>), it
would be hard to consider this a feature in most cases.</p>
<p>When you call your function as <code class="inline"><span class="i">&amp;foo</span><span class="s">(</span><span class="s">)</span></code>
, then you <i>do</i> get a new @_,
but prototyping is still circumvented.</p>
<p>Normally, you want to call a function using <code class="inline"><span class="i">foo</span><span class="s">(</span><span class="s">)</span></code>
.  You may only
omit the parentheses if the function is already known to the compiler
because it already saw the definition (<code class="inline"><a class="l_k" href="functions/use.html">use</a></code> but not <code class="inline"><a class="l_k" href="functions/require.html">require</a></code>),
625
or via a forward reference or <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">subs</span></code>
626 627 628 629
 declaration.  Even in this
case, you get a clean @_ without any of the old values leaking through
where they don't belong.</p>
<a name="How-do-I-create-a-switch-or-case-statement%3f"></a><h2>How do I create a switch or case statement?</h2>
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<p>If one wants to use pure Perl and to be compatible with Perl versions
prior to 5.10, the general answer is to write a construct like this:</p>
<pre class="verbatim">	for <span class="s">(</span><span class="i">$variable_to_test</span><span class="s">)</span> <span class="s">{</span>
		if    <span class="s">(</span><span class="q">/pat1/</span><span class="s">)</span>  <span class="s">{</span> <span class="s">}</span>     <span class="c"># do something</span>
		elsif <span class="s">(</span><span class="q">/pat2/</span><span class="s">)</span>  <span class="s">{</span> <span class="s">}</span>     <span class="c"># do something else</span>
		elsif <span class="s">(</span><span class="q">/pat3/</span><span class="s">)</span>  <span class="s">{</span> <span class="s">}</span>     <span class="c"># do something else</span>
		else            <span class="s">{</span> <span class="s">}</span>     <span class="c"># default</span>
		<span class="s">}</span></pre>
<p>Here's a simple example of a switch based on pattern matching,
lined up in a way to make it look more like a switch statement.
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We'll do a multiway conditional based on the type of reference stored
in $whatchamacallit:</p>
<pre class="verbatim">    SWITCH: for (ref $whatchamacallit) {</pre><pre class="verbatim">	<span class="q">/^$/</span>		&amp;&amp; <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;not a reference&quot;</span><span class="sc">;</span></pre>
<pre class="verbatim">	<span class="q">/SCALAR/</span>	&amp;&amp; <a class="l_k" href="functions/do.html">do</a> <span class="s">{</span>
				<span class="i">print_scalar</span><span class="s">(</span><span class="i">$$ref</span><span class="s">)</span><span class="sc">;</span>
				<a class="l_k" href="functions/last.html">last</a> <span class="j">SWITCH</span><span class="sc">;</span>
			<span class="s">}</span><span class="sc">;</span></pre>
<pre class="verbatim">	<span class="q">/ARRAY/</span>		&amp;&amp; <a class="l_k" href="functions/do.html">do</a> <span class="s">{</span>
				<span class="i">print_array</span><span class="s">(</span><span class="i">@$ref</span><span class="s">)</span><span class="sc">;</span>
				<a class="l_k" href="functions/last.html">last</a> <span class="j">SWITCH</span><span class="sc">;</span>
			<span class="s">}</span><span class="sc">;</span></pre>
<pre class="verbatim">	<span class="q">/HASH/</span>		&amp;&amp; <a class="l_k" href="functions/do.html">do</a> <span class="s">{</span>
				<span class="i">print_hash</span><span class="s">(</span><span class="i">%$ref</span><span class="s">)</span><span class="sc">;</span>
				<a class="l_k" href="functions/last.html">last</a> <span class="j">SWITCH</span><span class="sc">;</span>
			<span class="s">}</span><span class="sc">;</span></pre>
<pre class="verbatim">	<span class="q">/CODE/</span>		&amp;&amp; <a class="l_k" href="functions/do.html">do</a> <span class="s">{</span>
				<a class="l_k" href="functions/warn.html">warn</a> <span class="q">&quot;can&#39;t print function ref&quot;</span><span class="sc">;</span>
				<a class="l_k" href="functions/last.html">last</a> <span class="j">SWITCH</span><span class="sc">;</span>
			<span class="s">}</span><span class="sc">;</span></pre>
<pre class="verbatim">	<span class="c"># DEFAULT</span></pre>
<pre class="verbatim">	<a class="l_k" href="functions/warn.html">warn</a> <span class="q">&quot;User defined type skipped&quot;</span><span class="sc">;</span></pre>
661
<pre class="verbatim">    }</pre><p>See <a href="perlsyn.html">perlsyn</a> for other examples in this style.</p>
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<p>Sometimes you should change the positions of the constant and the variable.
For example, let's say you wanted to test which of many answers you were
given, but in a case-insensitive way that also allows abbreviations.
You can use the following technique if the strings all start with
different characters or if you want to arrange the matches so that
one takes precedence over another, as <code class="inline"><span class="q">&quot;SEND&quot;</span></code>
 has precedence over
<code class="inline"><span class="q">&quot;STOP&quot;</span></code>
 here:</p>
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<pre class="verbatim">	<a class="l_k" href="functions/chomp.html">chomp</a><span class="s">(</span><span class="i">$answer</span> = &lt;&gt;<span class="s">)</span><span class="sc">;</span>
	if    <span class="s">(</span><span class="q">&quot;SEND&quot;</span>  =~ <span class="q">/^\Q$answer/i</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Action is send\n&quot;</span>  <span class="s">}</span>
	elsif <span class="s">(</span><span class="q">&quot;STOP&quot;</span>  =~ <span class="q">/^\Q$answer/i</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Action is stop\n&quot;</span>  <span class="s">}</span>
	elsif <span class="s">(</span><span class="q">&quot;ABORT&quot;</span> =~ <span class="q">/^\Q$answer/i</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Action is abort\n&quot;</span> <span class="s">}</span>
	elsif <span class="s">(</span><span class="q">&quot;LIST&quot;</span>  =~ <span class="q">/^\Q$answer/i</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Action is list\n&quot;</span>  <span class="s">}</span>
	elsif <span class="s">(</span><span class="q">&quot;EDIT&quot;</span>  =~ <span class="q">/^\Q$answer/i</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Action is edit\n&quot;</span>  <span class="s">}</span></pre>
677
<p>A totally different approach is to create a hash of function references.</p>
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<pre class="verbatim">	<a class="l_k" href="functions/my.html">my</a> <span class="i">%commands</span> = <span class="s">(</span>
		<span class="q">&quot;happy&quot;</span> <span class="cm">=&gt;</span> \<span class="i">&amp;joy</span><span class="cm">,</span>
		<span class="q">&quot;sad&quot;</span><span class="cm">,</span>  <span class="cm">=&gt;</span> \<span class="i">&amp;sullen</span><span class="cm">,</span>
		<span class="q">&quot;done&quot;</span>  <span class="cm">=&gt;</span> <a class="l_k" href="functions/sub.html">sub</a> <span class="s">{</span> <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;See ya!&quot;</span> <span class="s">}</span><span class="cm">,</span>
		<span class="q">&quot;mad&quot;</span>   <span class="cm">=&gt;</span> \<span class="i">&amp;angry</span><span class="cm">,</span>
	<span class="s">)</span><span class="sc">;</span></pre>
<pre class="verbatim">	<a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;How are you? &quot;</span><span class="sc">;</span>
	<a class="l_k" href="functions/chomp.html">chomp</a><span class="s">(</span><span class="i">$string</span> = <span class="q">&lt;STDIN&gt;</span><span class="s">)</span><span class="sc">;</span>
	if <span class="s">(</span><span class="i">$commands</span>{<span class="i">$string</span>}<span class="s">)</span> <span class="s">{</span>
		<span class="i">$commands</span>{<span class="i">$string</span>}-&gt;<span class="s">(</span><span class="s">)</span><span class="sc">;</span>
	<span class="s">}</span> else <span class="s">{</span>
		<a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;No such command: $string\n&quot;</span><span class="sc">;</span>
	<span class="s">}</span></pre>
<p>Note that starting from version 5.10, Perl has now a native switch
statement. See <a href="perlsyn.html">perlsyn</a>.</p>
<p>Starting from Perl 5.8, a source filter module, <code class="inline"><span class="w">Switch</span></code>
, can also be
used to get switch and case. Its use is now discouraged, because it's
not fully compatible with the native switch of Perl 5.10, and because,
as it's implemented as a source filter, it doesn't always work as intended
when complex syntax is involved.</p>
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<a name="How-can-I-catch-accesses-to-undefined-variables%2c-functions%2c-or-methods%3f"></a><h2>How can I catch accesses to undefined variables, functions, or methods?</h2>
<p>The AUTOLOAD method, discussed in <a href="perlsub.html#Autoloading">"Autoloading" in perlsub</a> and
<a href="perltoot.html#AUTOLOAD%3a-Proxy-Methods">"AUTOLOAD: Proxy Methods" in perltoot</a>, lets you capture calls to
undefined functions and methods.</p>
<p>When it comes to undefined variables that would trigger a warning
704
under <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">warnings</span></code>
705
, you can promote the warning to an error.</p>
706
<pre class="verbatim">	<a class="l_k" href="functions/use.html">use</a> <span class="w">warnings</span> <span class="w">FATAL</span> <span class="cm">=&gt;</span> <span class="q">qw(uninitialized)</span><span class="sc">;</span></pre>
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<a name="Why-can't-a-method-included-in-this-same-file-be-found%3f"></a><h2>Why can't a method included in this same file be found?</h2>
<p>Some possible reasons: your inheritance is getting confused, you've
misspelled the method name, or the object is of the wrong type.  Check
out <a href="perltoot.html">perltoot</a> for details about any of the above cases.  You may
also use <code class="inline"><a class="l_k" href="functions/print.html">print</a> <a class="l_k" href="functions/ref.html">ref</a><span class="s">(</span><span class="i">$object</span><span class="s">)</span></code>
 to find out the class <code class="inline"><span class="i">$object</span></code>
 was
blessed into.</p>
<p>Another possible reason for problems is because you've used the
716
indirect object syntax (eg, <code class="inline"><span class="w">find</span> <span class="w">Guru</span> <span class="q">&quot;Samy&quot;</span></code>
717 718 719 720 721
) on a class name
before Perl has seen that such a package exists.  It's wisest to make
sure your packages are all defined before you start using them, which
will be taken care of if you use the <code class="inline"><a class="l_k" href="functions/use.html">use</a></code> statement instead of
<code class="inline"><a class="l_k" href="functions/require.html">require</a></code>.  If not, make sure to use arrow notation (eg.,
722
<code class="inline"><span class="w">Guru</span><span class="w">-&gt;find</span><span class="s">(</span><span class="q">&quot;Samy&quot;</span><span class="s">)</span></code>
723 724 725 726 727 728 729
) instead.  Object notation is explained in
<a href="perlobj.html">perlobj</a>.</p>
<p>Make sure to read about creating modules in <a href="perlmod.html">perlmod</a> and
the perils of indirect objects in <a href="perlobj.html#Method-Invocation">"Method Invocation" in perlobj</a>.</p>
<a name="How-can-I-find-out-my-current-package%3f"></a><h2>How can I find out my current package?</h2>
<p>If you're just a random program, you can do this to find
out what the currently compiled package is:</p>
730
<pre class="verbatim">	<a class="l_k" href="functions/my.html">my</a> <span class="i">$packname</span> = <span class="w">__PACKAGE__</span><span class="sc">;</span></pre>
731 732 733
<p>But, if you're a method and you want to print an error message
that includes the kind of object you were called on (which is
not necessarily the same as the one in which you were compiled):</p>
734 735 736 737 738
<pre class="verbatim"><a name="amethod"></a>	sub <span class="m">amethod</span> <span class="s">{</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$self</span>  = <a class="l_k" href="functions/shift.html">shift</a><span class="sc">;</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$class</span> = <a class="l_k" href="functions/ref.html">ref</a><span class="s">(</span><span class="i">$self</span><span class="s">)</span> || <span class="i">$self</span><span class="sc">;</span>
		<a class="l_k" href="functions/warn.html">warn</a> <span class="q">&quot;called me from a $class object&quot;</span><span class="sc">;</span>
		<span class="s">}</span></pre>
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<a name="How-can-I-comment-out-a-large-block-of-perl-code%3f"></a><h2>How can I comment out a large block of perl code?</h2>
<p>You can use embedded POD to discard it.  Enclose the blocks you want
to comment out in POD markers.  The &lt;=begin&gt; directive marks a section
742
for a specific formatter.  Use the <code class="inline"><span class="w">comment</span></code>
743 744 745
 format, which no formatter
should claim to understand (by policy).  Mark the end of the block
with &lt;=end&gt;.</p>
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<pre class="verbatim">	<span class="c"># program is here</span></pre>
<pre class="verbatim">	=<span class="w">begin</span> <span class="w">comment</span></pre>
<pre class="verbatim">	<span class="w">all</span> <span class="w">of</span> <span class="w">this</span> <span class="w">stuff</span></pre>
<pre class="verbatim">	<span class="w">here</span> <span class="w">will</span> <span class="w">be</span> <span class="w">ignored</span>
	<span class="w">by</span> <span class="w">everyone</span></pre>
<pre class="verbatim">	=<span class="w">end</span> <span class="w">comment</span></pre>
<pre class="verbatim">	=<span class="w">cut</span></pre>
<pre class="verbatim">	<span class="c"># program continues</span></pre>
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<p>The pod directives cannot go just anywhere.  You must put a
pod directive where the parser is expecting a new statement,
not just in the middle of an expression or some other
arbitrary grammar production.</p>
<p>See <a href="perlpod.html">perlpod</a> for more details.</p>
<a name="How-do-I-clear-a-package%3f"></a><h2>How do I clear a package?</h2>
<p>Use this code, provided by Mark-Jason Dominus:</p>
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<pre class="verbatim"><a name="scrub_package"></a>	sub <span class="m">scrub_package</span> <span class="s">{</span>
		<a class="l_k" href="functions/no.html">no</a> <span class="w">strict</span> <span class="q">'refs'</span><span class="sc">;</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$pack</span> = <a class="l_k" href="functions/shift.html">shift</a><span class="sc">;</span>
		<a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;Shouldn't delete main package&quot;</span>
			if <span class="i">$pack</span> eq <span class="q">&quot;&quot;</span> || <span class="i">$pack</span> eq <span class="q">&quot;main&quot;</span><span class="sc">;</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$stash</span> = <span class="i">*</span>{<span class="i">$pack</span> . <span class="q">'::'</span>}{<span class="w">HASH</span>}<span class="sc">;</span>
		<a class="l_k" href="functions/my.html">my</a> <span class="i">$name</span><span class="sc">;</span>
		foreach <span class="i">$name</span> <span class="s">(</span><a class="l_k" href="functions/keys.html">keys</a> <span class="i">%$stash</span><span class="s">)</span> <span class="s">{</span>
			<a class="l_k" href="functions/my.html">my</a> <span class="i">$fullname</span> = <span class="i">$pack</span> . <span class="q">'::'</span> . <span class="i">$name</span><span class="sc">;</span>
			<span class="c"># Get rid of everything with that name.</span>
			<a class="l_k" href="functions/undef.html">undef</a> <span class="i">$$fullname</span><span class="sc">;</span>
			<a class="l_k" href="functions/undef.html">undef</a> <span class="i">@$fullname</span><span class="sc">;</span>
			<a class="l_k" href="functions/undef.html">undef</a> <span class="i">%$fullname</span><span class="sc">;</span>
			<a class="l_k" href="functions/undef.html">undef</a> <span class="i">&amp;$fullname</span><span class="sc">;</span>
			<a class="l_k" href="functions/undef.html">undef</a> <span class="i">*$fullname</span><span class="sc">;</span>
776
	<span class="s">}</span>
777
	<span class="s">}</span></pre>
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<p>Or, if you're using a recent release of Perl, you can
just use the Symbol::delete_package() function instead.</p>
<a name="How-can-I-use-a-variable-as-a-variable-name%3f"></a><h2>How can I use a variable as a variable name?</h2>
<p>Beginners often think they want to have a variable contain the name
of a variable.</p>
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<pre class="verbatim">	<span class="i">$fred</span>    = <span class="n">23</span><span class="sc">;</span>
	<span class="i">$varname</span> = <span class="q">&quot;fred&quot;</span><span class="sc">;</span>
	++<span class="i">$$varname</span><span class="sc">;</span>         <span class="c"># $fred now 24</span></pre>
786 787 788 789 790 791 792
<p>This works <i>sometimes</i>, but it is a very bad idea for two reasons.</p>
<p>The first reason is that this technique <i>only works on global
variables</i>.  That means that if $fred is a lexical variable created
with my() in the above example, the code wouldn't work at all: you'd
accidentally access the global and skip right over the private lexical
altogether.  Global variables are bad because they can easily collide
accidentally and in general make for non-scalable and confusing code.</p>
793
<p>Symbolic references are forbidden under the <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">strict</span></code>
794 795 796 797 798 799 800 801 802 803
 pragma.
They are not true references and consequently are not reference counted
or garbage collected.</p>
<p>The other reason why using a variable to hold the name of another
variable is a bad idea is that the question often stems from a lack of
understanding of Perl data structures, particularly hashes.  By using
symbolic references, you are just using the package's symbol-table hash
(like <code class="inline"><span class="i">%main::</span></code>
) instead of a user-defined hash.  The solution is to
use your own hash or a real reference instead.</p>
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<pre class="verbatim">	<span class="i">$USER_VARS</span>{<span class="q">&quot;fred&quot;</span>} = <span class="n">23</span><span class="sc">;</span>
	<span class="i">$varname</span> = <span class="q">&quot;fred&quot;</span><span class="sc">;</span>
	<span class="i">$USER_VARS</span>{<span class="i">$varname</span>}++<span class="sc">;</span>  <span class="c"># not $$varname++</span></pre>
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<p>There we're using the %USER_VARS hash instead of symbolic references.
Sometimes this comes up in reading strings from the user with variable
references and wanting to expand them to the values of your perl
program's variables.  This is also a bad idea because it conflates the
program-addressable namespace and the user-addressable one.  Instead of
reading a string and expanding it to the actual contents of your program's
own variables:</p>
814 815
<pre class="verbatim">	<span class="i">$str</span> = <span class="q">'this has a $fred and $barney in it'</span><span class="sc">;</span>
	<span class="i">$str</span> =~ <span class="q">s/(\$\w+)/$1/eeg</span><span class="sc">;</span>		  <span class="c"># need double eval</span></pre>
816 817
<p>it would be better to keep a hash around like %USER_VARS and have
variable references actually refer to entries in that hash:</p>
818
<pre class="verbatim">	<span class="i">$str</span> =~ <span class="q">s/\$(\w+)/$USER_VARS{$1}/g</span><span class="sc">;</span>   <span class="c"># no /e here at all</span></pre>
819 820 821
<p>That's faster, cleaner, and safer than the previous approach.  Of course,
you don't need to use a dollar sign.  You could use your own scheme to
make it less confusing, like bracketed percent symbols, etc.</p>
822 823
<pre class="verbatim">	<span class="i">$str</span> = <span class="q">'this has a %fred% and %barney% in it'</span><span class="sc">;</span>
	<span class="i">$str</span> =~ <span class="q">s/%(\w+)%/$USER_VARS{$1}/g</span><span class="sc">;</span>   <span class="c"># no /e here at all</span></pre>
824 825 826 827 828
<p>Another reason that folks sometimes think they want a variable to
contain the name of a variable is because they don't know how to build
proper data structures using hashes.  For example, let's say they
wanted two hashes in their program: %fred and %barney, and that they
wanted to use another scalar variable to refer to those by name.</p>
829 830 831 832
<pre class="verbatim">	<span class="i">$name</span> = <span class="q">&quot;fred&quot;</span><span class="sc">;</span>
	<span class="i">$$name</span>{<span class="w">WIFE</span>} = <span class="q">&quot;wilma&quot;</span><span class="sc">;</span>     <span class="c"># set %fred</span></pre>
<pre class="verbatim">	<span class="i">$name</span> = <span class="q">&quot;barney&quot;</span><span class="sc">;</span>
	<span class="i">$$name</span>{<span class="w">WIFE</span>} = <span class="q">&quot;betty&quot;</span><span class="sc">;</span>	<span class="c"># set %barney</span></pre>
833 834
<p>This is still a symbolic reference, and is still saddled with the
problems enumerated above.  It would be far better to write:</p>
835 836
<pre class="verbatim">	<span class="i">$folks</span>{<span class="q">&quot;fred&quot;</span>}{<span class="w">WIFE</span>}   = <span class="q">&quot;wilma&quot;</span><span class="sc">;</span>
	<span class="i">$folks</span>{<span class="q">&quot;barney&quot;</span>}{<span class="w">WIFE</span>} = <span class="q">&quot;betty&quot;</span><span class="sc">;</span></pre>
837 838 839 840 841 842
<p>And just use a multilevel hash to start with.</p>
<p>The only times that you absolutely <i>must</i> use symbolic references are
when you really must refer to the symbol table.  This may be because it's
something that can't take a real reference to, such as a format name.
Doing so may also be important for method calls, since these always go
through the symbol table for resolution.</p>
843
<p>In those cases, you would turn off <code class="inline"><span class="w">strict</span> <span class="q">&#39;refs&#39;</span></code>
844 845
 temporarily so you
can play around with the symbol table.  For example:</p>
846 847 848 849 850
<pre class="verbatim">	<span class="i">@colors</span> = <span class="q">qw(red blue green yellow orange purple violet)</span><span class="sc">;</span>
	for <a class="l_k" href="functions/my.html">my</a> <span class="i">$name</span> <span class="s">(</span><span class="i">@colors</span><span class="s">)</span> <span class="s">{</span>
		<a class="l_k" href="functions/no.html">no</a> <span class="w">strict</span> <span class="q">'refs'</span><span class="sc">;</span>  <span class="c"># renege for the block</span>
		<span class="i">*$name</span> = <a class="l_k" href="functions/sub.html">sub</a> <span class="s">{</span> <span class="q">&quot;&lt;FONT COLOR='$name'&gt;@_&lt;/FONT&gt;&quot;</span> <span class="s">}</span><span class="sc">;</span>
	<span class="s">}</span></pre>
851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866
<p>All those functions (red(), blue(), green(), etc.) appear to be separate,
but the real code in the closure actually was compiled only once.</p>
<p>So, sometimes you might want to use symbolic references to directly
manipulate the symbol table.  This doesn't matter for formats, handles, and
subroutines, because they are always global--you can't use my() on them.
For scalars, arrays, and hashes, though--and usually for subroutines--
you probably only want to use hard references.</p>
<a name="What-does-%22bad-interpreter%22-mean%3f"></a><h2>What does "bad interpreter" mean?</h2>
<p>(contributed by brian d foy)</p>
<p>The "bad interpreter" message comes from the shell, not perl.  The
actual message may vary depending on your platform, shell, and locale
settings.</p>
<p>If you see "bad interpreter - no such file or directory", the first
line in your perl script (the "shebang" line) does not contain the
right path to perl (or any other program capable of running scripts).
Sometimes this happens when you move the script from one machine to
867
another and each machine has a different path to perl--/usr/bin/perl
868 869 870 871 872 873 874 875 876 877 878
versus /usr/local/bin/perl for instance. It may also indicate
that the source machine has CRLF line terminators and the
destination machine has LF only: the shell tries to find
/usr/bin/perl&lt;CR&gt;, but can't.</p>
<p>If you see "bad interpreter: Permission denied", you need to make your
script executable.</p>
<p>In either case, you should still be able to run the scripts with perl
explicitly:</p>
<pre class="verbatim">	% perl script.pl</pre><p>If you get a message like "perl: command not found", perl is not in
your PATH, which might also mean that the location of perl is not
where you expect it so you need to adjust your shebang line.</p>
879 880 881 882
<a name="REVISION"></a><h1>REVISION</h1>
<p>Revision: $Revision: 10100 $</p>
<p>Date: $Date: 2007-10-21 20:59:30 +0200 (Sun, 21 Oct 2007) $</p>
<p>See <a href="perlfaq.html">perlfaq</a> for source control details and availability.</p>
883
<a name="AUTHOR-AND-COPYRIGHT"></a><h1>AUTHOR AND COPYRIGHT</h1>
884
<p>Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909
other authors as noted. All rights reserved.</p>
<p>This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.</p>
<p>Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain.  You are permitted and
encouraged to use this code in your own programs for fun
or for profit as you see fit.  A simple comment in the code giving
credit would be courteous but is not required.</p>
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