use strict;
use XML::Rules;

my $parser = XML::Rules->new(
	style => 'filter',
	rules => {
		_default => 'raw',
		'id,gpa' => 'raw extended',
		student => sub {
			my ($tag, $attrs, $parser) = @_[0,1,4];
			my $id = $attrs->{':id'}{_content};
			if (exists $parser->{parameters}{ $id }) {
				$attrs->{':gpa'}{_content} = $parser->{parameters}{ $id };
			}
			return $tag => $attrs;
		}
	}
);

$parser->filter( \*DATA, \*STDOUT, {'002' => 'B'});


__DATA__
<students>
<student>
<name>John</name>
<id>001</id>
<gpa>A</gpa>
</student>
<student>
<name>John</name>
<id>002</id>
<gpa>C</gpa>
</student>
</students>