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

my $input_xml = 'big.xml';

# tags to keep
my $re = qr/u_application_i|u_ci_id|sys_class_name|name|ip_address/;
my $twig = 'XML::Twig'->new( 
  twig_handlers =>  { result => \&record }, 
);
open my $fh,'>','small.xml' or die "$!";
$twig->parsefile($input_xml);

sub record {
  my ($e,$t) = @_;
  for ($t->children){
    $_->cut unless ($_->name =~ $re);
  }
  $t->flush($fh,pretty_print => 'indented');
}

##</code><code>##

<result>
    <ip_address>10.10.10.10</ip_address>
    <u_application_id></u_application_id>
    <u_display_name>lnxnmsprag27</u_display_name>
    <name>lnxnmsprag27</name>
    <u_ci_id>CI10110212</u_ci_id>
    <sys_class_name>cmdb_ci_linux_server</sys_class_name>
 </result>