When you fork your process, the parent and child don't have any better way of talking to each other than any other two programs on the system (i.e., signals, reading/writing the same files). You can fork via open instead, in which case you'll get a (one-way) file handle to talk through, but it sounds as if you want two-way communication (send a newline, get back the output). For that, look at IPC::Open3 or IPC::Run.