I think it's a shame that they gave up on the 5.0005 style threading
Well, lots of very smart people decided it was impossible to make work safely.
Note that the linux kernel is a different beast. It has a few well-defined entry points, and it is (relatively) easy to make concurrency initially fairly restrictive, and gradually increase it as code is audited and reworked. With the perl internals, you have XS code that can more or less do anything to anything in memory. Every single thing in the perl internals (and XS code) must be completely thread-safe before you can allow threads to take charge.
(Ok I'm simplifying a bit, but you get the general idea.)
Anyway, I'm bored of discussing this now,