perl -MFile::Find -le 'for (@INC) {
    find(sub { print $File::Find::name if /pm$/ }, $_)
}'
[download]

Update: here's one that cleans up the output to resemble the module names, rather than the .pm filenames:

perl -MFile::Find -le '
for $inc (@INC) {
  find(sub {
             return unless /pm$/;

             ($module = $File::Find::name) =~ s!^$inc/!!;
             $module =~ s!/!::!g;
             $module =~ s/.pm$//;

             print $module
           },
       $inc)
}'
[download]