There is another older Perl threading flavour called the 5.005 model, unsurprisingly for 5.005 versions of Perl.
The old model is known to have problems, deprecated, and will probably be removed around release 5.10.
It tends to keep the individual tasks small and simple, as well as allowing some parts of the pipeline to block (on I/O or system calls, for example) while other parts keep going.
If you're running different parts of the pipeline on different processors you may also take advantage of the caches on each processor.
Prime and Fibonacci generators both map well to this form of the pipeline model.
The user-level interface to ithreads (the the threads manpage classes) appeared in the 5.8.0 release, and as of this time is considered stable although it should be treated with caution as with all new features. For many non-trivial threaded programs you'll need to choose different models for different pieces of your program.
A thread is a flow of control through a program with a single execution point. The boss/worker model usually has one ``boss'' thread and one or more ``worker'' threads.
Threads are a new feature, and even some of the standard modules aren't thread-safe.
Even if a module is thread-safe, it doesn't mean that the module is optimized to work well with threads.