||= happens in Perl code, yet it's not thread safe.

You're still not getting it.

• ||= is thread-safe, just as with any other operation--unless it is applied to a shared variable.

  That's the basic tenet of threads. Everything(*) is thread-safe unless it involves explicitly shared variables. Clear and unambiguous.

  It is the very raison d'etre of ithreads.

• Any Perl operation applied to any shared variable is not thread-safe unless you take locks.

  That the basic tenet threads::shared. Again, clear and unambiguous.

There is no purpose in documenting the thread-safety of every opcode--or internal operations--because they are all completely covered by those two simple tenets

And that is what sets iThreads apart, and above, any other shared-state concurrency implementation. You use it exactly as if it was non-threaded, with no shared-state, even for the nasties of interpreted language threading--fat variables & closures. Except where you need shared-state, then you mark it explicitly and take the appropriate precautions.

Sure, the implementation could be more efficient--of time and memory. And the user could be given finer-grained control. But for the most part, inventing a model that allows so many pre-threading modules to just work inside threads is a remarkable achievement. Even the majority of non-Pure Perl modules function perfectly within iThreads, and that's truly remarkable, and no accident.

Artur Bergman should be lauded.

(*)For the pedants: External state excluded.