I think your problem is the needed delay, as Corion pointed out.

However, I was just looking this over offline and noticed that you have another "redundant" IO::Select->new() statement in your process_errors sub. I'm not sure if it affects your performance, but it isn't needed. You already have *cmdErr added to your select in main, so all you need to do is the sysread in your sub. It may "gum up the works" in IO::Select. ?