Not quite.

Yes, you're right. Thanks for correcting.
I had tested for USE_ITHREADS=undef USE_MULTI=define USE_IMP_SYS=undef and found it to fail.
However, I'm now finding that configuration to work fine - so the original test was obviously not done correctly.

From memory, that original test that I did failed in exactly the same way as USE_ITHREADS=undef USE_MULTI=undef USE_IMP_SYS=define !!
(I wonder what could possibly have gone wrong ;-)

Cheers,
Rob