use XML::Twig;
open my $ofh, '>', \my $outxml or die $!;
my $twig = XML::Twig->new(
	twig_print_outside_roots => $ofh,
	twig_roots => {
		'/includes/include/priority' => sub {
			my ($twig, $elt) = @_;
			$elt->set_text( $elt->text+1 );
			$elt->flush($ofh);
		},
	} );
$twig->parse($xml);
close $ofh;
print $outxml;