use strict;
use XML::Simple;

my $data = XMLin(
  './data.xml', keyattr => {field => 'name'}, forcearray => [ qw(field number required) ]
);


$data->{fields}->{field}->{foo}->{number}->[0] += 10;
$data->{fields}->{field}->{bar}->{number}->[0] += 10;
  
print XMLout($data, keyattr => {field => 'name'}, rootname => 'data');