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package Animal;
use base qw(Class::Virtual);
...
__PACKAGE__->virtual_methods(qw(get_sound get_color));

1;

use strict;
use warnings;
...
warn 'Animal::Duck forgot to implement ' . join ', ', @missing if @mis
+sing;

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

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