1) You subcommand2.pl test is IMHO a false positive.

Indeed, but my intention was to proof that it at least can read something and not hang (so, exec() call works like expected etc).

2) As soon as you modify your main.pl to the following, it works

Damn! You are right! I was trying version which closes READHANDLE, but forgot about WRITEHANDLE ! Thank you. I think it's solved now!

p.s.
I upvoted your sarcastic post, it looks acceptable to me.