"Class data"

package A;
use Moose;

has 'data' => ( is => 'ro', isa => 'Str', default => 'OH HAI' );

package B; # can't name a package B in real life
use Moose;
extends 'A';

has '+data' => ( default => 'OH HAI FROM B!' );

A->new->data # OH HAI
B->new->data # OH HAI FROM B!