package Base;
sub new { bless \my($scalar), shift }

package Optional;
use Carp qw(croak);

sub optional_method {
    croak q{You said you would implement optional_method and didn't!};
}

package A;
use base qw(Base); # No optional_method.

package B;
use base qw(Base Optional); # Must implement optional_method.

sub optional_method { 'Woot!' }