use XML::SAX::ExpatXS;
use WDSTreeHandler;
...
my $tree = read_file("/x/y/z/aap.xml");
my $handler = WDSTreeHandler->new;
my $parser = XML::SAX::ExpatXS->new(Handler => $handler);
$parser->parse_string($tree);

##</code><code>##

package WDSTreeHandler;
use base qw(XML::SAX::Base);
...
sub start_document
{
	my ($self, $doc) = @_;
}

sub start_element
{
	my ($self, $el) = @_;
	...
	if ($level >= 0 && $el->{'LocalName'} =~ /folder|leaf/)
	{
		## do some stuff and try to STOP PARSING
	}
}
sub end_element
{
	...
}
sub end_document
{
}
1;