The reason you need select or else IO::Select is very simple. When you are both talking to and listening from a process it is very important that you are not talking when it is trying to talk, or listening when it is trying to listen. If that happens you'll get a deadlock which lasts forever.

What select (either version) does is tell you whether it is ready to talk or receive more input, so that you can correctly send or receive data. That way you know which you can safely do and avoid deadlocks.

Please note that doing this correctly is complicated. If at all possible you want to arrange to need to talk to an external process, or listen from it, but not both.