package Animal;
use base qw(Class::Virtual);
use strict;
use warnings;

sub new {

    my $class = shift;

    # hash reference
    my $self = shift;
    bless $self, $class;
    return $self;

}

__PACKAGE__->virtual_methods(qw(get_sound get_color));

1;

##</code><code>##

use strict;
use warnings;
use Animal::Dog;
use Animal::Duck;

my $dog  = Animal::Dog->new(  { name => 'Atila' } );
my $duck = Animal::Duck->new( { name => 'Donald' } );

print $dog->get_name(),  ' says ', $dog->get_sound(),  "\n";

my @missing = Animal::Duck->missing_methods();
warn 'Animal::Duck forgot to implement ' . join ', ', @missing if @missing;

print $duck->get_name(), ' says ', $duck->get_sound(), "\n";

##</code><code>##

C:\>test.pl
Atila says "bark bark!"
Animal::Duck forgot to implement get_sound at C:\test.pl line 12.
Animal::Duck forgot to implement get_sound() at C:\test.pl line 14