Editing Perl module (section)

==Perl packages and namespaces==
A running Perl program has a built-in [[namespace]] called "<code>main</code>", which is the default name. For example, a subroutine called <code>Sub1</code> can be called as <code>Sub1()</code> or <code>main::Sub1()</code>. With a variable the appropriate [[Sigil (computer programming)|sigil]] is placed in front of the namespace; so a scalar variable called <code>$var1</code> can also be referred to as <code>$main::var1</code>, or even <code>$::var1</code>. Other namespaces can be created at any time.
<syntaxhighlight lang="perl">
package Namespace1;
$var1 = 1;	# created in namespace Namespace1, which is also created if not pre-existing
our $var2 = 2;	# also created in that namespace; our required if use strict is applied
my $var3 = 3;	# lexically-scoped my-declared - NOT in any namespace, not even main
</syntaxhighlight>

<syntaxhighlight lang="perl">
$Namespace2::var1 = 10; # created in namespace Namespace2, also created if not pre-existing
our $Namespace2::var2 = 20; # also created in that namespace
my $Namespace2::var3 = 30;#compilation error:my-declared variables CAN'T belong to a package
</syntaxhighlight>

Package declarations apply package scope till the next package declaration or the end of the block in which the declaration is made.
<syntaxhighlight lang="perl">
our $mainVar = 'a';
package Sp1;
our $sp1aVar = 'aa';
print "$main::mainVar\t$sp1aVar\n"; # note mainVar needs qualifying
package Sp2;
our $sp2aVar = 'aaa';
print "$main::mainVar\t$Sp1::sp1aVar\t$sp2aVar\n";# note mainVar and sp1aVar need qualifying
package main;
print "$mainVar\t$Sp1::sp1aVar\t$Sp2::sp2aVar\n"; # note sp1aVar and sp2aVar need qualifying

$mainVar = 'b';
{
    # NOTE previously created packages and package variables still accessible
    package Sp1;
    our $sp1bVar = 'bb';
    print "$main::mainVar\t$sp1aVar\t$sp1bVar\n"; # note mainVar needs qualifying
    {
        package Sp2;
        our $sp2bVar = 'bbb';
        print "$main::mainVar\t$Sp1::sp1aVar$Sp1::sp1bVar\t$sp2aVar$sp2bVar\n";
    }		# note mainVar and sp1...Var need qualifying
    print "$main::mainVar\t$sp1bVar$sp1aVar\t$Sp2::sp2bVar$Sp2::sp2aVar\n";
}		# note package Sp1 applies by default
# main applies again by default; all package variables still accessible as long as qualified
print "$mainVar\t$Sp1::sp1aVar$Sp2::sp2bVar\n";
</syntaxhighlight>

===Packages and modules===
Conventionally, namespaces are associated with modules; in practice, there is usually one namespace per module and vice versa, but that's not mandated by the language. For example, the 'standard' module CGI.pm has the following declaration at its top:
<syntaxhighlight lang="perl">
package CGI;
</syntaxhighlight>
This module, and its functionality, would commonly be invoked as follows:
<syntaxhighlight lang="perl">
use CGI (':standard'); # imports many functions, including b()
...
print b('Hello, world'); # outputs <b>Hello, world</b>
</syntaxhighlight>
A 'missing' subroutine ''could'' be added from the using program's namespace.
<syntaxhighlight lang="perl">
sub CGI::bi { # define target namespace (CGI) and sub name (bi)
    return b(i($_[0]));
}
</syntaxhighlight>
and invoked as below:
<syntaxhighlight lang="perl">
print CGI::bi('Hello, world'); # outputs <b><i>Hello, world</i></b>
</syntaxhighlight>
However, though technically feasible, that would be dubious programming practice. You might just as well define the sub in the calling namespace, and call it from that namespace.