gistfile1.pl

use strict;
use warnings;
use Test::More tests => 2;

do {
    package Class1;
    use Mouse;

    has A => (
        is  => 'ro',
        default => 'A',
    );

    has B => (
        is  => 'ro',
        default => sub { $_[0]->A . ':B' },
    );

    package Class2;
    use Mouse;

    has B => (
        is  => 'ro',
        default => 'A',
    );

    has A => (
        is  => 'ro',
        default => sub { 'A:' . $_[0]->B },
    );
};

my $obj1 = Class1->new;
is $obj1->B, 'A:B';

my $obj2 = Class2->new;
is $obj2->A, 'A:B';