The problem here is that you flush the in-memory tree exactly twice: once when you hit the record you want to update, and once at the very end, when the file is all parsed.

In your case, what you need is to use the twig_roots option, which will only process the 1 record you want to update, with the (awfully named) twig_print_outside_roots option, that will output, untouched, everything else:

my $twig= new XML::Twig( 
                twig_roots =>                  
                  { 'Person[@id="50000"]' => \&Person },
                twig_print_outside_roots => 1, 
                       );
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You don't need the last flush, at the end of the code. And I thought that you would have to replace the call to flush in the sub body by a call to $person->print, but it looks like flush is smart enough to do what you mean, so no change needed there.

Does this help?