Inheritance ...

package Point3D;
use Moose;

extends 'Point';

has 'z' => (is => 'rw', isa => 'Int', default => 0);

override 'clear' => sub {
    my $self = shift;
    super();
    $self->z(0);
};

# or ...

after 'clear' => sub {
    my $self = shift;
    $self->z(0);
};