I like having a contract so I'd mix the method in through the magic of multiple inheritance.

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!' }
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You get ->can('optional_method') now but you also have your code croak if the contract isn't met. Or you can put shared code in the super Optional::optional_method. Your choice.