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    <p>It's covered in <a href="http://perldoc.perl.org/perlmod.html" rel="nofollow">Perlmod</a>.</p> <p>The <code>::</code> is really a <em>namespace</em> identifier. In early Perl before this idea of namespace took hold, you could have variable name collisions happening in your program:</p> <p>Here's my program:</p> <pre><code>#! /usr/bin/env perl #use strict; use warnings; use feature qw(say); require "test2.pl"; $foo = "bar"; futz_with_foo(); say $foo; </code></pre> <p>I set <code>$foo</code> to <em>bar</em> and never touched it. Should print out <code>bar</code> However, when I run my program, I get:</p> <pre><code>$ ./test.pl WOO WOO! I MESSED WITH FOO! </code></pre> <p>In <code>test2.pl</code>, I have:</p> <pre><code>sub futz_with_foo { $foo = "WOO WOO! I MESSED WITH FOO!" } 1; </code></pre> <p>What happened is that both programs are using the same variable <code>$foo</code>. To get around this issue Perl gives modules the ability to have their own namespace. The original syntax was the single quote, but changed to <code>::</code> in Perl 4 (if I remember correctly). You can still use the single quote. You declare your namespace with the <a href="http://perldoc.perl.org/fuctions/package.html" rel="nofollow">package</a>.</p> <p>The best way to understand this is to see it in action. Try the following:</p> <pre><code>#! /usr/bin/env perl use strict; use warnings; use feature qw(say); our $foo = 'This is the value of $foo'; say '$foo: ' . $foo; say '$main::foo: ' . $main::foo; say "\$main'foo: " . $main'foo;; say "\nSwitching to package Bar"; package Bar; our $foo = 'This is in package Bar'; say '$foo: ' . $foo; say '$Bar::foo: ' . $Bar::foo; say "\nSwitching to package main again"; package main; say '$foo: ' . $foo; </code></pre> <p>Running this, I get:</p> <pre><code>$foo: This is the value of $foo $main::foo: This is the value of $foo $main'foo: This is the value of $foo Switching to package Bar $foo: This is in package Bar $Bar::foo: This is in package Bar Switching to package main again $foo: This is in package Bar </code></pre> <p>By default, your program starts out in the main <em>namespace</em>. By the way, you'll notice that I declared <code>our $foo</code> and not <code>my $foo</code>. That's because <code>our</code> puts the variable in the Perl symbol table where <em>package</em> variables are stored. The <code>my</code> is a lexically scoped declaration, and is now preferred. A variable declared with <code>my</code> only exists in its declared scope and that can't be outside of the file its in.</p> <p>Maybe this will shed some light on the error message you get when you forget to declare a variable with <code>my</code>:</p> <pre><code>Global symbol "$foo" requires explicit package name at ... </code></pre> <p>By default, all Perl variables are <em>Package</em> variables (that is, they're in Perl's symbol table). The <code>use strict</code> pragma forces you to either declare package variables with <code>our</code> or forces you to use the full package name of the variable. By declaring a variable with <code>my</code> (like we do 99 40/100% of the time) and by using <code>strict</code>, we force you to declare your variables when using Perl.</p> <p>Hope this helps.</p>
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