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use Animal;
use Cow;
...
Cow->speak;
Horse->speak;
Sheep->speak;

package Animal;

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

package Cow;

...
our @ISA = qw(Animal);

sub sound { "moooo" }

package Sheep;

...
our @ISA = qw(Animal);

sub sound { "baaaah" }

package Horse;

...
our @ISA = qw(Animal);

sub sound { "neigh" }

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