Are you running this program under the CGI? If so then yes you will get that message because you are not outputting any headers at all. You should first try to run this from the command line to see if it works, then if you want to run it as a CGI program you should add:
print "Content-Type: text/plain\n\n";
into the program somewhere before entering the foreach loop.
Of course Net::Whois::IP is not the module that you want at all because that module is designed to query for WHOIS records for IP blocks - you probably want to look at either Net::Whois or Net::Whois::Raw. For example:
use CGI qw(:standard);
use Net::Whois::Raw qw(whois);