You have to problems with your arguments passed to gethostbyaddr. First, you are correct that $& is not in the correct format. You must pass it in packed address format, which can be done using Socket's inet_aton to convert the address from readable dotted form to network-ordered bytes. (Perl's "version strings" appear to generate a similarlly formatted string.) Secondly, the second arg to gethostbyaddr needs to be a valid type of address, which would be AF_INET, probably. You do not want 'tcp', because that is a protocol, not a type of address. (In fact, TCP can and does have different types of addresses, with IPv6.)

Note that it is a good idea to use capturing parenthesis instead of $&, because capturing parenthesis tend to make your program faster. (See also perlvar.)