TIMTOWTDI using split and RegExp

use strict;
use warnings;
my $data = do{local $/; <DATA>};

my ($header, $class, $footer) = $data =~ m/^(.*<\/header>)(.*)(<footer
+>.*)$/is;
my @data = split/(?=<class>)/, $class;
my %hash;

for my $data (@data){
    if ($data =~ m/<strength>((?:(?!<\/strength>).)*)<\/strength>/si){
        my $str = $1;
        if (defined($hash{$str})) { $hash{$str} .= $data }else{ $hash{
+$str} = $data}
  }
}

my @sort = sort{$a <=> $b} (keys %hash);

print "$header\n";
print $hash{$_} for (@sort);
print "$footer";

__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>
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