Threads within a process all share the same address space.

Is that true on every platform? Will that always be true, even in a next version or Perl?

I don't think Perl gives you that promise;

Yes. Yes. And it is not within the auspices of Perl to make or break that promise. They would not be threads otherwise, as that is the very definition of threads.

I conceed that it is possible that someone could implement something and call it "threads" that would not satisfy that definition, but it would be a very obtuse thing to do.