#! /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;
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or, less verbose XML::XSH2 :

open file.xml ;
for /includes/include set priority (priority + 1) ;
save :b ;
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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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