The following does what you want, although I don't think it's particularly pretty. I think that XML::Twig (as recommended by Cristoforo), or at least XML::LibXML would be much better. Note that both XML::XPath and XML::Twig use the same underlying parser, XML::Parser, which in turn uses Expat.

use warnings;
use strict;
use XML::XPath;

my $xml = <<'ENDXML';
<includes>
    <include>
        <priority>9990</priority>
        <required>true</required>
        <location>/efs/dist/tpcommon/itrstemplates/latest/common/templ
+ates/equities.xml</location>
    </include>
    <include disabled="true">
        <priority>10001</priority>
        <required>true</required>
        <location>/efs/dist/itrs/tools/latest/common/templates/global_
+authentication.xml</location>
    </include>
</includes>
ENDXML

my $xp = XML::XPath->new(xml => $xml);
my $nodes = $xp->find('//includes/include');
for my $include ($nodes->get_nodelist) {
    for my $p ($xp->findnodes('./priority', $include)) {
        my $priority = $p->string_value;
        $p->removeChild($_) for $p->getChildNodes;
        $p->appendChild( XML::XPath::Node::Text->new($priority+1) );
    }
}
my ($root) = $xp->findnodes('/');
print $root->toString, "\n";
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In regards to the last two lines above, I haven't yet found an easy way to print back out the XML document, maybe I'm missing something obvious. Anyway, for comparison, here's an XML::Twig solution:

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;
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I've written some things a little longer than they need to be in this code for clarity, the code could be shortened quite a bit, and also the output doesn't need to go to a string, it could be sent to a file directly.

I'm not really acquainted with XML, but this node suggests (in one of the replies), that XML::XPath is better suited to finding but not editing.

He ended up using XML::Twig.

Do you want to increase all priorities or just some? If all then something like this would be enough:

use strict;
use XML::Rules;

my $filter = XML::Rules->new(
style => "filter",
rules => {
  priority => sub { 'priority' => $_[1]{_content}+1 },
});

$filter->filter(\*DATA, \*STDOUT);
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If you need to change only some priorities, you can add some tests:

my $filter = XML::Rules->new(
  style => "filter",
  ident => '    ',
  rules => {
    _default => 'as is',
    include => sub {
      if ($_[1]{location}{_content} =~ m{/itrstemplates/}) {
        $_[1]{priority}{_content}++;
      }
      delete $_[1]{_content}; # remove extra whitespace
      return include => $_[1]
    },
  }
);
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#! /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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