The Class::Contract module implements strong encapsulation, static
inheritance, and design-by-contract condition checking for
object-oriented Perl.  The module provides a declarative syntax for
attribute, method, constructor, and destructor definitions at both the
object and class level.  Pre-conditions, post-conditions, and class
invariants are also fully supported.

WWW: http://search.cpan.org/search?dist=Class-Contract

-- Sergey Skvortsov
skv@FreeBSD.org