Well you could try this:

use strict;
use XML::Simple;

my $xs = XML::Simple->new( 
  forcearray => 1, keyattr => {}, rootname => 'data'
);

my $data = $xs->XMLin('./data.xml');

foreach my $field (@{$data->{fields}->[0]->{field}}) {
  $field->{number}->[0] += 10;
}
  
print $xs->XMLout($data);
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Or this:

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');
[download]

Update: plus the obligatory link to Does your XML::Simple code pass the strict test?