use Animal;
use Cow;
use Horse;
use Sheep;

Cow->speak;
Horse->speak;
Sheep->speak;
##</code><code>##

package Animal;

use 5.006;
use strict;
use warnings;

sub speak
{
    my $class = shift;
    print "a $class goes ", $class->sound, "!\n";
}

sub sound
{
    die "'You have to define sound() in a subclass' found";
}

##</code><code>##

package Cow;

use 5.006;
use strict;
use warnings;
use Animal;
our @ISA = qw(Animal);

sub sound { "moooo" }

##</code><code>##

package Sheep;

use 5.006;
use strict;
use warnings;
use Animal;
our @ISA = qw(Animal);

sub sound { "baaaah" }

##</code><code>##

package Horse;

use 5.006;
use strict;
use warnings;
use Animal;
our @ISA = qw(Animal);

sub sound { "neigh" }

##</code><code>##

a Cow goes moooo!
a Horse goes neigh!
a Sheep goes baaaah!