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

use XML::Twig;
 
XML::Twig->parse( pretty_print => 'indented', shift @ARGV)
         ->root
         ->sort_children( \&get_pui, type => 'numeric')
         ->print;

sub get_pui
  { my( $item)= @_;
    return $item->first_descendant( 'itemid[@idtype="PUI"]')->text;
  }