There are only two threads/interpreters and the thread that cloned an interpter does nothing else but wait for the right event to invoke the callback. So, bit confused what could be the 'wrong' interprter ... because BGThread cannot call code into an interpreter it did not create (i.e perl.exe).

We agree that there are two interpreters. We agree that it is important that the "right" interpreter be used for the callback.

The problem is, if the callback is triggered by an asynchronous event, how can you guarantee that only right interpreter is running when the callback occurs?

Because if you don't take steps to ensure that, then it will inevitably happen that sometimes the triggering event will occur during the time-slice of the wrong interpreter, and it will segfault.

Have you tried my suggestion of logging the current thread id from a) the place where you get the Perl callback address; b) where you invoke that Perl callback address?