#!/usr/bin/perl

use Net::DNS
use Net::Netmask;

my $resolver = Net::DNS::Resolver->new;
print $_, "\n" for grep
{
    ! $resolver->query( $_, 'PTR' )
}
ips( '81.86/16' );

sub ips
{
    my $net = Net::Netmask->new(@_);
    wantarray ? $net->enumerate : \@{$net->enumerate};
}