(Answer reposted here.)

Checking and setting a value in a hash has to be faster than hitting the filesystem. Here is a script that does this:

use strict;
use File::Find;

my %seen;
sub wanted {
  if (-d $File::Find::name) {
    if ($seen{$File::Find::name}) {
      $File::Find::prune = 1;
      return;
    }
    $seen{$File::Find::name} = 1;
  }
  print "$File::Find::name\n" if /\.pm$/;
}

find \&wanted, @INC;
[download]

Note that the wanted function should check %seen only for actual directories. Setting $File::Find::prune is what actually stops File::Find from recursing further into the directory.