I have no doubt that is possible. To an extent, this is already done in threads.pm (because different subroutines are "imported" depending on whether you are using a threaded or an unthreaded perl).
But this doesn't solve the migration path problem! Code written with the 5.6.x version of forks.pm won't work in 5.8.x without change. I'm trying to find a solution that will allow a smooth upgrade path, so that code does not have to be changed. I believe using a source filter when running in 5.6.x will solve this problem. I'm working on that this weekend. Hopefully there'll be an update again to the original node when it's uploaded to CPAN.