So I ran a few tests. It seems that the child can't read from the parent's STDIN. I presume this is the problem to which you are referring.

man 2 setpgrp explains

At any time, one (and only one) of the process groups in the session can be the foreground process group for the terminal; the remaining process groups are in the background.

I don't know what a session is, but adding setsid() appears to solve the problem.

In reply to Re^3: Killing process group run with IPC::Open3 by ikegami
in thread Killing process group run with IPC::Open3 by pm_sanchay

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