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);
[download]

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]
    },
  }
);
[download]