I assume you are attempting to use some non-blocking I/O, correct? As far as I know, ActivePerl does not support F_SETFL at all, at least I have never been able to get it to work in the past, and my colleagues tell me the same. You are likely going to have to resort to another method altogether. You might look into IPC::Multiplex. I have not used this myself, but recall many references to it when I was looking at a similar problem. Hope this helps.