I think the $twig->purge is a little too strong for what you are doing. It seems to interfere with the way the twig is stored. Using $element->delete instead when the element is not one you want to output would work better.

I will investigate some more to understand better what's going on, but in the mean time a handler like this seems to work:

sub parseEntry 
  { my ($twig,$element)=@_;
    my $id=$element->att("id");
    if ($id eq "OVAL3") 
      { $twig->flush(); 
        return 1; 
      } 
    else
      { $element->delete; }
    return 0; 
  }
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