. . . doing a whois query against the online database . . .

That is the only way I can think of to do it unless you have a central database of all registered domains that you can check against. If you want to keep the noise down, you could split off the work into some child processes, giving each one a part of the list of names to check against. Each child process would query a different name server, thus spreading the load out.

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I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated