Duh ... anyone with half a brain would probably be sceptical that MCF threads are going to just magically work with perl without any need to modify the perl source.
The fact that my "mcf" build has a runtime dependency on libmcfgthread-1.dll was enough to fool me - but I think that my "mcf" build might very well be a "pthreads" one, and the difference I'm seeing is the result of a change between gcc-12.2.0 and gcc-13.0.1.
I'll work on reproducing the difference in a C program and take it up with the appropriate body.