Your problem statement's still a little vague, and you'd hit snags with people who have an A record for their domain and/or international domains (e.g. "foo.com" or "foo.co.uk"). You might get close by using URI::URL to parse them and then try and look up an NS record with Net::DNS (strip components and stopping when you hit a "top level" server such as "com" or "co.uk").