in reply to
Child process reading <STDIN> from parent
Why do you want to do that in the first place? Not that there aren't perfectly good reason to try and write on another process' STDIN, but since you don't seem to know very well what you are doing, I can't help but wonder if you didn't take that problem by the wrong end.
Then, to answer your question, from what little information you gave us, it looks like a job for pipes. In perlopentut you'll see how to open a process and obtain a filehandle to read from its output or write in its input.