gistfile1.txt

package Base;
use Moose;
has 'base' => ( is => 'ro' , isa => 'Str' , default => 'base' );

package App;
use Moose;
extends 'Base';
has 'app' => ( is => 'ro' , isa => 'Str' , default => 'app' );

package App::DB;
use Moose;
extends 'App';
has 'db' => ( is => 'ro' , isa => 'Str' , default => 'db' );

package App::DB::Mysql;
use Moose;
extends 'App::DB';
has '+db' => ( default => 'mysql' );

package App::DB::PostgreSQL;
use Moose;
extends 'App::DB';
has '+db' => ( default => 'postgres' );

package main;
use strict;
use warnings;
use 5.010;

my $db = App::DB->new;
my $my = App::DB::Mysql->new;
my $pg = App::DB::PostgreSQL->new;

say $_->db foreach ( $db , $my , $pg );