A computer is a state machine. Threads are for people who can't program state machines. -- Alan Cox

I'm assuming this is the Alan Cox in question ? I suppose its all well and good for someone who hacks OS kernels for fun and profit to make such statements. However, as someone who has also hacked kernels (including of the realtime, SMP kind) for fun and profit, I'd adjust Mssr. Cox's assertion a bit:

Threads are for people who can't have better things to do than program state machines.

However, if Thread::Apartment is as capable as current testing indicates, then I'll agree with your assertion that "there's no good reason for lib authors to make their libs thread-safe". Because they won't have to, assuming they're reasonably OO Perl. Just pop them into an apartment thread, and call the methods and/or invoke its closures as needed.

Yes I realize there are issues apartment threading can't solve. But I've managed to get some threads-hostile DBI drivers to behave, and hope to have Tk working soon, which indicates many otherwise threads hostile modules should be supportable.