#!/usr/bin/perl

# http://perlmonks.org/?node_id=1208859

use strict;
use warnings;

my $awsLists = 'd.1208859'; # change to your filename

my @commands = <DATA>; # new ips (with \n)

my %old;
if( open my $in, '<', $awsLists ) # OK if file not there
  {
  @old{ <$in> } = (); # note: keys have \n
  close $in;
  }

my $oldsize = keys %old;
@old{ @commands } = (); # add new ips

if( keys %old > $oldsize ) # only write if new ips added
  {
  open my $out, '>', $awsLists or die "$! opening $awsLists";
  print $out sort keys %old; # sort not needed, but nice touch :)
  close $out;
  }

__DATA__
ip-10-1-129-212.us-west-2.compute.internal
ip-10-1-129-168.us-west-2.compute.internal
ip-10-1-129-212.us-west-2.compute.internal
ip-10-1-129-158.us-west-2.compute.internal
ip-10-1-129-78.us-west-2.compute.internal
ip-10-1-1-145.us-west-2.compute.internal
ip-10-1-65-215.us-west-2.compute.internal
ip-10-1-129-194.us-west-2.compute.internal
ip-10-1-1-252.us-west-2.compute.internal
ip-10-1-67-133.us-west-2.compute.internal