And of course here is the OXTV (Obligatory XML::Twig Version), using no variable except $_:

Update: jeffa noticed that this does not work with the CPAN version of XML::Twig, you will need the development version (3.10) to be able to chain the calls to set_content and to print (I changed set_content in this version to return the element). So with older versions you will need to do this:

  $_->set_content( $_->text + 10);
  $_->print;
[download]

instead of being able to ncely chain method calls. How lame! ;--(

#!/usr/bin/perl -w
use strict;
use XML::Twig;

XML::Twig->new( twig_roots => { number => sub { $_->set_content( $_->t
+ext + 10)
                                                  ->print;
                                              }
                              },
                twig_print_outside_roots => 1,
              )
         ->parse( \*DATA);

__DATA__
<data>
    <fields>
     <field name="foo">
        <number>2</number>
        <required>Y</required>
      </field>
       <field name="bar">
        <number>1</number>
        <required>N</required>
      </field>
    </fields>
</data>
[download]