my $fb = new Cow;
$fb->sound;

package Rooster;
use base 'Animal';
sub new
  {
    bless { animal_type => 'rooster',
            sound       => 'cluck' }, shift
  }

package Cow;
use base 'Animal';
sub new
  {
    bless { animal_type => 'cow',
            sound       => 'moo' }, shift
  }

package Animal;
sub sound
  {
    my $self = shift;
    print $self->{animal_type} . "s go: " . $self->{sound};
  }