#!/usr/bin/perl
use strict;
use warnings;
use XML::Twig;

my $header='u_ci_id,u_display_name,name,fqdn,ip_address,mac_address';
my $field = $ARGV[0] || 'u_ci_id';
my $input_xml = do { local $/; <DATA> };

my $twig = new XML::Twig;
$twig->xparse($input_xml);
my $root = $twig->root();

my @data = ();
my @cols = split ',',$header;
unshift @cols, $field; # added for sort

for my $record ($root->children) {
  push @data,[ map { $record->first_child($_)->text } @cols];
}

my @sorted = sort { $b->[0] cmp $a->[0] } @data;
print $header."\n";
for (@sorted){
  print join ',',@$_[1..$#cols]; # exclude sort col
  print "\n";
}

__DATA__
<?xml version="1.0" encoding="UTF-8"?>
<response>
 <result>
  <fqdn>fqdnB</fqdn>
  <ip_address>ip1</ip_address>
  <u_display_name>display1</u_display_name>
  <name>name1</name>
  <mac_address>mac5</mac_address>
  <u_ci_id>id 1</u_ci_id>
 </result>
 <result>
  <fqdn>fqdnA</fqdn>
  <ip_address>ip2</ip_address>
  <u_display_name>display2</u_display_name>
  <name>name2</name>
  <mac_address>mac4</mac_address>
  <u_ci_id>id 2</u_ci_id>
</result>
</response>