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

my (@sortedM, @sortedL);
my $i=0;
my $twig= new XML::Twig(twig_handlers => { 
								'enhancement' => sub {$_->first_child->sort_children_on_att("type");},
								'descriptor' => \&sortSubEltsHandler,
							},
						pretty_print => 'indented',	
			keep_atts_order => 1,
            );			
$twig->parse(\*DATA); 
$twig->print;
$twig->purge;

sub sortSubEltsHandler
{ 	
	my( $t,$elt)= @_;
	my $root= $t->root;	
	my @mainterms = $root->descendants(q{mainterm});
	   @sortedM = reverse sort { $a->text cmp $b->text }
		@mainterms;
	my @links = $root->descendants(q{link});
	   @sortedL =reverse sort { $a->text cmp $b->text }
		@links;
	$elt->cut_children;
	foreach my $mainterm (@sortedM) {
	my $new_elt= XML::Twig::Elt->new($mainterm->name => $mainterm->text);
	$new_elt->paste($elt);
	}
	foreach my $link (@sortedL) {
	my $new_elt= XML::Twig::Elt->new($link->name => $link->text);
	$new_elt->paste($elt);
	}
}#end of sortSubEltsHandler

__DATA__
<enhancement>
<descriptorgroup>
<descriptors type="MED" controlled="y">
<descriptor>
<mainterm weight="a">plague</mainterm>
<mainterm>disease</mainterm>
<mainterm>pestilence</mainterm>
<link>f</link>
<link>C</link>
<link>M</link>
</descriptor>
</descriptors>
<descriptors type="DRG" controlled="y">
<descriptor>
<mainterm>panadol</mainterm>
<mainterm weight="a">zorro</mainterm>
<mainterm>xenon</mainterm>
<mainterm weight="a">aspirin</mainterm>
<link>b</link>
<link>P</link>
<link>z</link>
</descriptor>
</descriptors>
</descriptorgroup>
</enhancement>