With due respect, I admit that this hack is cool and a source of great learning experience for others.
However, I have to ask: what is the client's requirement, to be more exact? Prior to the time of this post, there had already been JSON::PP and Mojo::JSON (which became JSON::Tiny in Oct 2012) on CPAN. They are both faster than this regex version, already support Unicode, and I suspect have smaller footprint also. JSON::PP is also much preferable to me because it gives helpful error message instead of just returning undef or die-ing with 'no match'. You can just stick either in the project source repo and be done with it in a couple of minutes.