Well Reusable threads demo shows a pool of threads working on an array. Your problem will be to keep your network objects contained in the threads (threadsafe), and to clear out the previous run's data, before starting on the next server. Much depends on what modules you will be using in your threads for the portscanning, and whether it is threadsafe.

You may be better off using forks instead of threads. With a fork, you will be able to handle timeouts easier, and memory cleanup is better. Since you just want results printed out, you can have each forked child print to a file. The only way threads would be useful, is if you need communication between threads in realtime, like thread3 is checking if thread4 is working ok. If you don't need that realtime interaction, forks are better. See Parallel::ForkManager.