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);