I've had good luck threading in Unix w/5.6.1, very bad luck on Windows. If you're trying to do any threading on Win32 beyond a couple simple forks(ithreads), I would highly suggest avoiding it.
It looks like it's still experimental. Info on 5.6 at Perl.com says:
Perl's threading has been drastically reworked. It's still experimental, and there's still every expectation that the current implementation may be thrown out and started again, but Perl now supports two different types of threads.
Granted, they are talking about 5.6.0, but I believe that's still valid.
Also, from perl5.6.1 perlthrtut documenation
WARNING: Threading is an experimental feature. Both the interface
and implementation are subject to change drastically. In fact, this
documentation describes the flavor of threads that was in version
5.005. Perl 5.6.0 and later have the beginnings of support for
interpreter threads, which (when finished) is expected to be
significantly different from what is described here. The information
contained here may therefore soon be obsolete. Use at your own risk!