Here's a solution that I cribbed from Randal Schwartz years ago and modified to my own tastes:

#!/usr/bin/perl -w
use strict;
use File::Find;

my $re = shift || ".";

find sub {
    return unless my ($x) = $File::Find::name =~ m{\./(.*\.pm)$};
    $x =~ s,/,::,g;
    print "$x\n" if /\Q$re\E/i;
}, map "$_/.", grep -d && /^[^.]/, @INC;
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If you just invoke it by name, you get the entire list of installed modules. If you specify a regex as an argument, you'll get only the modules that match the regex (case-insensitive). I use it pretty regularly.