use strict;
use warnings;
no warnings 'uninitialized';

use XML::Rules;

my $parser = XML::Rules->new(
	style => 'filter',
	rules => {
		_default => 'raw',
		itemid => sub {
			my ($tag,$attrs,$context,$parents) = @_;
			$parents->[-4]{':PUI'} = $attrs->{_content} if $attrs->{idtype} eq "PUI";
			return [$tag => $attrs]; # same thing the 'raw' built-in does
		},
		item => 'as array',
		bibdataset => sub {
			my ($tag,$attrs) = @_;
			@{$attrs->{item}} = sort {$a->{':PUI'} <=> $b->{':PUI'}} @{$attrs->{item}};
			$attrs->{_content} = [
				(map( ( "\n\t", [item => $_]), @{$attrs->{item}})),
				"\n",
			];
			delete $attrs->{item};
			return $tag => $attrs;
		},
	}
);

$parser->filter(\*DATA);

__DATA__
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<bibdataset ...