I'd like to know how did you check that there is that interference. Perhaps it was just output buffering or some other coincidence (to have non buffered output you need to add $| = 1 to your scripts). I think it is not possible for Unix processes to directly interfere one with another (not using some IPC technique). Beside that you are not using threads - you might confuse people when you talk about interfering threads and mean interfering child processes.