use strict;
use XML::Rules;

my $parser = XML::Rules->new(
	rules => {
		_default => 'content',
		'^residues' => sub {
			my ($tag,$attr,$context,$parents) = @_;
			return ($context->[-1] eq 'feature' and $parents->[-1]{feature_name} eq 'feature_one');
		},
		'residue' => sub {
			my ($tag,$attr) = @_;
			print "$attr->{residue_index} $attr->{residue_name}\n"
		},
	}
);
$parser->parse(\*DATA);

__DATA__
<sas_residue_annotation xmlns="http://url/Schema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://url/Schema test.xsd">
...