I was wrong, Coro::AnyEvent::poll really helps. This is the same as sleep(0). I was pulling his leg, Coro’s author, shame on me. I just could not figure it out that Coro can be used without IO in practice And those using IO have Coro::AnyEvent::poll. Still, I learnt that Marc Lehmann is a patient and very polite person. I got a good lesson too: don’t make the output on STDIN artificially.