I don't use stackoverflow much and couldn't comment there, which might have been more appropriate, but I took Ikegami's solution to your stackoverflow post and modified it just a little as below and the result seemed to do what you wanted with one pass.

my $tee = IO::Tee->new($frufile, $vegfile);

my $twig = XML::Twig->new(
... etc.
    twig_print_outside_roots => $tee,
...
);
$twig->parse( *DATA );
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