At first glance it looks like a bug in XML::Twig, but if you see the test suite you'll see the use-case, XML-Twig-3.44-new/t/test_3_26.t 

      XML::Twig->new( twig_handlers => { bar => sub { $_->set_tag( 'to
+to')->flush; }})
               ->parsefile_inplace( $file, '.bak');

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The call to flush does printing (as well as eating of the tree), which you don't have

If you add  twig_handlers => { _all_ => sub { $_[0]->flush } } you'll get what you're after

In reply to Re: XML::Twig parsefile_inplace misunderstanding (xml_pp twig_handlers _all_ flush) by Anonymous Monk
in thread XML::Twig parsefile_inplace misunderstanding by Discipulus

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