Not that this has anything to do with Perl really. But anyway, what can you do:

1. A hostname A can give you an IP-number if it resolves.
2. A reverse lookup of the IP-number may give you a hostname B if it resolves.
3. A and B may or may not be the same.

You should think of hostnames as nothing other than a "roadmap" to get to a specific (IP) address. And there are an infinite number of ways to Rome: some of them are described (have a hostname) and some are not described. But you will never be able to find them all, unless someone (else) tells them to you.

Hope this helps.

Liz