#! /usr/bin/perl
use warnings;
use strict;

use XML::LibXML;

my $doc = 'XML::LibXML'->load_xml(location => shift);

for my $include ($doc->findnodes('/includes/include')) {
    my $priority = $include->find('priority/text()')->[0];
    $priority->setData($priority->data + 1);
}
print $doc;