package Cow;
use base qw(Animal);
__PACKAGE__->makes_sound_like('moo');
...

##</code><code>##

package Animal;
sub makes_sound_like {
  my $class = shift;
  my $sound = shift;
  our %sound_registry;
  if (exists $sound_registry{$sound}) {
    croak "two animals want the sound $sound\n";
  } else {
    $sound_registry{$sound} = $class;
  }
}