#!/usr/bin/perl -w
use strict;
use warnings;

use XML::Twig;

my $t= XML::Twig->new(pretty_print=>'indented');

$t->parse( join('',<DATA>) );

$t->root->sort_children( sub{ 
				my $e = shift;
				return 0 if $e->tag eq 'header';
				return 10000000 if $e->tag eq 'footer';
				my @strength = $e->getElementsByTagName('strength') or die "Malformed element:\n".$e->sprint;
				return $strength[0]->text;
		}, 
		type=>'numeric' );

$t->print;

__DATA__
<?xml version="1.0" encoding="UTF-8"?>
<system>
<header></header>
<class>
 <name>Eight</name>
 <strength>8</strength>
</class>
<class>
 <name>Four</name>
 <strength>4</strength>
</class>
<class>
 <name>Ten</name>
 <strength>10</strength>
</class>
<class>
 <name>One</name>
 <strength>1</strength>
</class>
<footer></footer>
</system>