Thanks, this has been very helpful. If I read the source correctly, it works as follows: -- A "global" interpreter is created that runs the module initialization code. -- When a perl function is going to be called, the global interpreter is used in the non-threading case. In the threading case, if necessary, a thread-local interpreter is created by cloning the global one. So, in the non-threading case, I would have to perform initialization tasks like opening a database connection in the module init code. In the threading case, I have do to that in CLONE(). In the module init code, can I tell whether I'm in the threading case or not? It's probaby no harm besides a waste of resources to also do the full init there in the threading case, but I would like to avoid that if easily possible.
Threading is not done in debug mode (-X), start your freeradius in normal mode to test multithreading. I think this should be documented since one is always urged to test in debug mode.