Finally, I kept getting "process ended prematurely at /home/cwood/bin/pr2 line 46" in my more involved work elsewhere. I printed the parent's output (child's input) at the parent, and that helped me discover a stray "exit 0" leftover from when my child was a script forked with each execution.

Similarly a "print 123" without the "\n" would have caused an apparent IPC freeze if I was checking for /\n/ in the parent's input (child's output).

So two answers:

1. unbuffered output on the child
2. watch for leftovers from previous techniques on parent and child