corion thanks for the answer!

I suppose that “sharing of resources” in your answer means sharing resources between threads within one Windows process.

As threads is implemented using Windows threads, it has then the same problems as fork!? My conclusion is then that you need two Windows process to test a TCP-connection. Or is there any other possibility to use threads?

I have difficulties to understand what you should avoid using threads in Windows. Are there any documentation explaining threads in Windows from a Perl perspective? Which type of resources are problematic to shared between Windows threads?