OK. Your test is much shorter than mine :-)

I'd recommend adding threads->yield() (or sleep()) before the async for the child -- to ensure the "STDIN thread" has reached the <>. (There is a race condition there.)

... I assume your 5.10 can function w/o the use of semaphores ...

Yes. It doesn't seem to matter on my system whether the close STDIN runs before or after the "STDIN thread" reaches the <> (I've tried it both ways).

But I cannot say whether that's a 5.10 thing, or because I'm running a uni-processor (or both, or neither).