in reply to
Using Threading to speed up DNS Resolution
Yes, it's possible to thred it, even without threading in your own script. Look at the docs for NET::DNS.
The best way to speed up the script depends a lot on the input. Make sure that the IP's are unique, or cache the results (in other words, only look up each IP number ONCE).
Also note that a fast "looker-upper" is going to place a significant load on whatever DNS you're using, and that the admin of that DNS might come looking for you with a stick.