Threads are not really in 5.005 either, they were an experimental feature. Not just experimental, they were also unstable! 5.6 plays around with a slightly different threading model. You also shouldn't trust that.

Please, please, please do not lead people to believe that they can use threaded Perl. They should not. The first version of Perl that is likely to have a threading model you can depend on surviving and being maintained will be 6.0. Telling people otherwise is just inviting them to get burned!