Here is an idea, if you've got a model, you can write a generator, see UML to Perl

Say you created models using Moose (or one of the alternatives) and they use MooseX::Params::Validate ... and now you're providing a RESTful interface to these classes.

Well, grab UML::Class::Simple and generate some uml, then you feed said uml through ummf to generate whatever you're after, a Joose based validator and jQuery interface to your RESTful api, a perl interface to your RESTful api ...

But IMHO, JSON really doesn't need a web service description language other than english

It turns out JSON-RPC-1.1 is contemptible :) http://search.cpan.org/perldoc/JSON::RPC::Common#RANT