but I feel you will still be quite helpful. So, let me describe a bit more of the project

Then I'm going to need much more detail about the client API and the perl script that is using it.

You'd need to considerably expand these two steps of the description:

Client API spawns threads and sends msgs to server to 'do things'.

The threads will then call the approriate CCBs based on various msgs received from server.

Questions that come to mind:

• One (C) thread per server connection?
• Does that thread procedure sit in a block loop?
• One PCB per thread?
• What does the Perl script do whilst waiting for PCBs to be called?
• Is there only (ever?) one Perl thread?
• Can two C-threads attempt to call the same PCB? Concurrently?

in my real project too, the threads are created on the C side and not by Perl. So, it seems this italisized statement would not apply, right?

If there is only (ever?) one Perl thread in the client program, then there will only be one Perl context.

But, in that case, by using dTHX: within the CCBs prior to calling the PCBs, all the context issues should (I believe) be resolved. That is certainly the case in both my test code, and my corrected version of your updates to that code, on my OS.

The fact that your real code is failing, and your attempts to use my test code fail at compile-time, strongly suggests that your problems distinctly fall into the realms of "platform dependant". Hence, my strong reservations about my ability to help you further.

I could continue to try and re-create your circumstances based entirely upon your (to date, scant) descriptions, but if the underlying problem in your real code would not manifest itself on my platform, I am never going to be able to re-create your problems here. No matter how much effort I expend.