use strict;
use warnings;

use Tk;
use Net::IP;
use Net::DNS;

my $ip_field;
my $name_field;

my $resolver = Net::DNS::Resolver->new()

sub rlookup {
    my $ip = Net::IP->new($ip_field, 4);
    my $answer = $resolver->query($ip->reverse_ip(), 'PTR');
    my @names = $answer->{'answer'};
    if (@names) {
       $name_field = $names->[0]->{'ptrdname'};
    } else {
       $name_field = "[no associated name]";
    }
}

{
   my $mw = MainWindow->new;
   $mw->title("IP Lookup");
   $mw->Label(-text => "Enter an IP Address")->pack;
   $mw->Entry(-textvariable => \$a)->pack;
   $mw->Button(-text => "Lookup", -command => \&rlookup)->pack;
   $mw->Entry(-textvariable => \$b)->pack;
   MainLoop;
}