In any case now I can work on elegant ways to kill the threads ...

There is no "elegant way to kill a thread".

Your best solution, assuming it is possible, would be to enable a timeout on the ssh connection.

Your second best solution would be to use a timer/signal to interrupt the read. That is difficult (if not impossible), due to Perl SAFE SIGNALS.

The third best solution would be to set the socket non-blocking and only read when there is something available. (Ie.Poll!)

The problem with killing the thread -- assuming you discover a reliable mechanism for doing so(*) -- is that all the data you've already so carefully arranged to capture, would be discarded. Ie. As you are returning the captured data as the return value from the thread, you'll never see it if you kill the thread.

(*)The problem here is that to kill the thread, you first need to interrupt the read. And if you can interrupt the read, there is no need to kill the thread.