#!/usr/local/bin/perl -w
use strict;
use XML::Simple;
local $/=undef; 
my $input=<DATA>;
my $xml=XMLin($input, 'forcearray' => 1);
$xml->{'fields'}->[0]{'field'}{$_}{'number'}->[0] += 10 for keys %{$xml->{'fields'}->[0]{'field'}};
print XMLout($xml, 'rootname' => '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>
_END__
<data>
  <fields name="field">
    <bar>
     <number>11</number>
     <required>N</required>
    </bar>
    <foo>
      <number>12</number>
      <required>Y</required>
    </foo>
  </fields>
</data>