#!/usr/bin/perl -w

$ipaddressfile = "FILENAME-TO-PLAINTEXT-IPADDRESS-FILE";

@iplist = "";
$i = 0;

open(IPLISTFILE,"<$ipaddressfile");
while(<IPLISTFILE>) {
     $iplist[$i] = $_;
     $i++;
}
close(IPLISTFILE)l

foreach $ipaddr (@iplist) {
     @ns = `nslookup $ipaddr`;
     $ns[3] =~ m%.* (.*)%;
     print $ipaddr . " resolves to " . $1 . "\n";
}