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(), $/;
}
[download]