That you get these symptoms with PIDs are probably a feature of the way perl is doing things, rather than Windows. PIDs and TIDs are created in the same way by kernel (they are just different flavours of the same thing).

The pipe will only be destroyed when the last handle is closed. If another process has a handle open then the pipe will not be destroyed when the server dies. However, this does raise a simple solution to my race condition. If the client opens a handle to the server process before opening the pipe then it effectively creates a zombie, so the pid cannot be reused. Obviously tidying these handles will be important.