See "perldoc perlfaq3", How do I find which modules are installed on my system?

Apart from the methods mentioned there you can also use CPANPLUS to list all modules (though I forgot the exact mechanism).

Update: This is the CPANPLUS solution I sometimes use; I have no idea if it's available from the command line interface too:

use strict;
use warnings;
use CPANPLUS;

my $c = CPANPLUS::Backend->new();

my %seen;
for my $m ($c->installed()) {
    next if $seen{$m->package}++;
    print $m->package(), "\t\t", $m->version(), $/;
}
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I didn't see it in perlfaq3, so I'll just mention that I use perldoc perllocal in combination with corelist. It's not foolproof—for example, it doesn't play well with Module::Build-based modules—but it gives a pretty good idea of what's on my system.

Find all modules in all "@INC" directories:

    findpm
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what modules are available for use ?

Most of these

Also see this page for groupings of that list.

Update: I forgot to mention that Module::CoreList can list the modules that come with various version of Perl

See also: Script to List all Install Modules

#!/sw/bin/perl

use strict;
use warnings;
use ExtUtils::Installed;
use Data::Dumper;

my ($inst) = ExtUtils::Installed->new();
print Dumper my (@modules) = $inst->modules( "all" );
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There's also a wrong answer (Unix variant):

locate perl | grep -i ModuleCategory/ModuleName, maybe with a grep for 'pm$' added. If necessary, replace locate ... with find /usr/*/perl* /opt/perl* /usr/*/*/perl*, but that's a bit slower.

It misses some, it adds some, but if you rarely use the 'correct' methods (@INC as mentioned above or the FAQs), it's way faster to remember :). So don't forget about the lowtech approach.