Wise ones, has Mandrake switched from threaded to nonthreaded perl in their distro ?

Apparently so (execute perl -V:useithreads to see if its threaded or not).

Would this cause problems if I try to run their modules with a threaded perl ?

Absolutely. You're already experienced it, and it's called binary incompatibility. The two perls aren't binary compatible. Just like a perl compiled for windows is not binary compatible with a perl compiled for mandrake, modules compiled for one perl aren't compatible with the other.

Does this mean they now support only nonthreaded perl ?

That's what it looks like.