Bruteforce-Search free Domain

#! /usr/bin/perl
use strict;
use Net::DNS;

my $resolver = Net::DNS::Resolver->new;
   $resolver->nameservers("ns1.univie.ac.at");
   $resolver->dnssec(0);
my $tld=".at";
#my @letters = ('a' .. 'z', 0 .. 9);
my @letters = ('a' .. 'z');

foreach my $letter1 (@letters) {
   foreach my $letter2 (@letters) {
      foreach my $letter3 (@letters) {
        checkDomain($letter1.$letter2.$letter3.$tld);
      }
   }
}

sub checkDomain($) { my $domain = shift(@_);
    my $reply = $resolver->send($domain, "NS");
    print $domain . "\n" if ($resolver->errorstring =~ /NXDOMAIN/);
}