Your hash would read better with a singular name, $ns_record{'foo.org'} pronounced "ns_record of foo.org".

By that logic, arrays should be singular as well, because $addr[4] is pronounced "address number 4". Some consistency would be nice.

For hashes, I use plural when they're used as an array, and singular when they're used as a structure: %ns_records would be a list of records ($ns_records{$domain}), while %ns_record would be a record with named fields ($ns_record{'ptr'}).