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?

The semantics of threads are far easier to reason about, as closing a handle in one thread will close the handle for the complete process for example. When using fork(), this would be unexpected, but when using the fork emulation on Windows, the same happens. This is obvious when you keep in mind that on Windows, fork emulation is implemented through threads, but programmers using fork() do usually not come from Windows and don't think of fork behaving differently.