test_default.pl

#!/usr/bin/perl

package MyBase;
use Mouse;

has something => (
    isa => 'Str',
    is => 'ro',
    default => 'foo',
);

has lazything => (
    isa => 'Str',
    is => 'ro',
    required => 1,
    lazy => 1,
    default => sub {
        return $_[0]->something;
    },
);

package MyBase::Extended;
use Mouse;

extends 'MyBase';

has +something => (
    default => 'bar',
);

has +lazything => (
    default => sub {
        return $_[0]->something . ' (wrapped)';
    }
);

package main;
use strict;
use warnings;
use Test::More qw(no_plan);

my $obj = MyBase->new;
is $obj->lazything, 'foo';

my $obj2 = MyBase::Extended->new;
is $obj2->lazything, 'bar (wrapped)';