I did not know that pids in Windows were negative.

They're not. That's why negative numbers were used to id the pseudo-processes created by fork.

Do you know what causes this incompatibility?

The usual possibilities:

• A race condition. Not very likely here considering the simplicity of the code and the repeatability of the problem.
• Something that should only be called once is being called once for each thread.
• Something is called from one thread but expects to be called from another.

On the Perl side, every variable gets copied when you create a new thread. That makes Perl code more-or-less thread-safe by default. But that's not the case on the C component of XS modules. C variables are shared among threads "by default".

Workarounds:

• If you only need Win32::OLE in the parent, you could try loading it after all threads have been created. This will probably entail using require instead of use.
• If you only need Win32::OLE in the child, you could try loading it from the thread. This will probably entail using require instead of use.
• If you need Win32::OLE in both the parent and the child, it probably won't work based on the evidence we've seen. Using separate processes would do the trick.