It's only a small piece of code, and if it works for you, why bother? I find all questions of the form "can this be done 'better'", or "what is the 'best' module to use when doing X" awkward. It all depends on your definition of "best". Some people say "best" means, "fastest", or "uses the least memory". For both types, the answer would usually be "do it in C instead of Perl". If with best you mean "least amount of key strokes", you should direct your question to a mailinglist about golf.

All I can say is how I would code it. Whether that is "better" or not, I leave to you. Afterall, it's your code, you get to decide what is better.

#!/usr/bin/perl

use strict;
use warnings;
use Net::DNS;

my $res = Net::DNS::Resolver -> new;
my $net = "81.86";

foreach my $c (0x00 .. 0xFF) {
    foreach my $d (0x00 .. 0xFF) {
        my $ip = "$net.$c.$d";
        print $ip unless $res -> query ($ip => 'PTR')
    }
}

__END__
[download]

I used 0x00 .. 0xFF instead of 0 .. 255 purely for aesthetic values.

Abigail