hoge.pl

use strict;
use warnings;
use utf8;
use 5.10.1;

package Parent {
    use Mouse;

    no Mouse;

    my $added;
    sub BUILD {
        my ($self) = @_;
        if (!$added->{$self->meta->name}) {
            $self->meta->add_around_method_modifier(as_string => sub {
                my $orig = shift;
                my $result = $orig->(@_);

                return 'extended_'.$result;
            });
            $added->{$self->meta->name}++;
        }
    }

    sub as_string {
        return "parent";
    }

    say Parent->new->as_string; #=> extended_parent
}

package Child {
    use Mouse;
    extends 'Parent';
    no Mouse;

    sub as_string {
        return 'child';
    }

    say Child->new->as_string; #=> extended_child
    say Child->new->as_string; #=> extended_child
    say Child->new->as_string; #=> extended_child
    say Child->new->as_string; #=> extended_child
}