I would use Threads::Queue to send messages back from your threads to your main thread instead of polling them for becoming ->joinable.

For passing thread results back otherwise, see threads, which documents the return value of ->join(). I would avoid using that, because to get more speed, you will want to keep a pool of worker threads and pass the queries to them using another Thread::Queue instead, thus saving the cost of creating and destroying a new thread for every request.