package Animal; #parent class, in file lib/Animal.pm 
sub new {
  return bless {}, shift;
}
sub sound {
  die "cannot call this method on class '".ref(shift)."'";
}
sub output_sound {
  my ($self, $what) = @_;
  print $what."\n";
}
1;

package Animal::Dog; #desc. of Animal, lib/Animal/Dog.pm
use base 'Animal';
sub sound {
  my ($self) = @_;
  $self->output_sound("haf");
}
1;

package Animal::Dog::Sheepdog; #desc. of Dog
                               #lib/Animal/Dog/Sheepdog.pm
use base 'Animal::Dog';
sub guard {
  my ($self, $flock) = @_;
  #do something
}
1;

package Animal::Pig; #other descendant
use base 'Animal';
sub sound {
  my ($self) = @_;
  $self->output_sound("khro");
}
1;