So, maybe you want to do something with $thr, then, if you want to monitor the newly created thread? The threads documentation lists various uses for the return value of ->create(). Maybe now is a good time to read that documentation and look for these uses.

Also, again, maybe you really don't want to ->detach threads but to just keep them around and ->join them. Just feed them from a Thread::Queue and wait until they have all finished as all work has been done. But then, I'm sure that approach has already been shown to you.