while (defined($line = <INPUT>)) {

    if ($line =~ /\b\d*\.\d*\.\d*\.\d*\b/) {
        $name = gethostbyaddr($&, 'tcp');

        if (defined($name)) {
            print "hostname for address $&: $name\n";

        }else{
            print "Couldn't get hostname for: $&\n";
        }
    }
}