Unfortunately, apart from the technical difficulties (I would appreciate it if someone could tell me how to make the PROTOTYPE: conditional in the XS code, depending on the perl version), this does not address the source incompatibility issue with the real threads module. It's not just about share(), it's also about lock(), cond_wait(), cond_signal() and cond_broadcast().

I don't see forks.pm as a substitute for the real threads module, just as a teaching and prototyping tool. I would like people to be able to try threaded programming by just downloading a CPAN module, rather than having to upgrade/rebuild perl. I would like the source incompatibility issue to be as small as possible.

That's why I'm considering the last option as the most favorable: if you want to try out threads programming in 5.6.x, then you know you will have to change your code when you upgrade to 5.8.x and/or real threads. If you want to try out threads programming in (unthreaded) 5.8.x, then you won't need to change anything when you upgrade. So, 5.6.x support is for really toying around, higher perls support a real upgrade path.

Liz