use XML::Rules;

my $parser = XML::Rules->new(
	rules => [
		_default => 'content',
		book => sub {
			return unless $_[1]->{category} eq $_[4]->{parameters};
			my $title = delete $_[1]->{title};
			delete $_[1]->{'_content'};
			$title => $_[1],
		},
		bookstore => 'pass no content',
	]
);

my $category = $ARGV[0] or die "Usage: BookStore2.pl category\n";

my $data = $parser->parse(\*DATA, $category);

use Data::Dumper;
print Dumper( $data);

__DATA__
<?xml version="1.0" encoding="ISO-8859-1"?>
<bookstore>
...