Object::InsideOut provides comprehensive support for implementing classes
using the inside-out object model.

This module implements inside-out objects as anonymous scalar references that
are blessed into a class with the scalar containing the ID for the object
(usually a sequence number). For Perl 5.8.3 and later, the scalar reference is
set as read-only to prevent accidental modifications to the ID. Object data
(i.e., fields) are stored within the class's package in either arrays indexed
by the object's ID, or hashes keyed to the object's ID.