You can also use executable acceptance tests that are written in the domain language of the contract. It might not be the actual "contract", but half way between unit tests and the contract.
I would recomment using Ruby Cucumber
http://github.com/aslakhellesoy/cucumber
But since you are a Perl shop, then maybe you can use my own small attempt at p5-cucumber.
http://github.com/kesor/p5-cucumber
