I think you may still be trying to pass a bit too much back and forth. Thread::Queue is a lovely way of handling queuing, but it works best with single values. You're passing a hash into probe_volume - which works single threaded, but can get quite complicated if multithreading.

I think you need to step back a little and consider the design - threading increases throughput by parallelism, but as a result means that each of your threads occur asynchronously and non deterministically - you will never know which order your threads will complete tasks in. You therefore can't do something like 'print probe_volume' - you'll have to collate your data and (potentially) reorder it first.

You will also need to think about sharing variables - you pass a hash into probe_volume, and return a list. This will probably cause you pain. Sharing variables between threads is potentially quite complicated and a source of some really annoying bugs. Try to avoid doing it.

I would therefore suggest that what you want is a 'standalone' probe_volume subroutine that takes _just_ a volume name (either passed via sub call, but ideally 'fed' through a Thread::Queue). And outputs (again, returning via sub call, or Thread::Queue) the results, but without using anything from the global namespace. (Read only access to e.g. command definitions would be ok)