So if you make two passes through libxml you get valid xml? But then you lose the vertical tab ...

If I try using toFile I get  error : xmlEscapeEntities : char out of range

Error seems to be coming from libxml2 itself ... this says sanitize your inputs first so ascii control chars aren't in there :/

I'd report it to XML::LibXML maintainer , for the clues he might provide :)

So if you make two passes through libxml you get valid xml?

No. I make one pass and use something (libxml in this case) to parse the result. If a result was produced, I'd expect the second tool to accept it.

Sanitizing input seems reasonable, but for such a complex data format, something like

$text = 'XML::LibXML::Text'->removeInvalidChars($str, $version)
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would be handy.
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

No. I make one pass ... removeInvalidChars

Um,

sub XML::LibXML::Node::toSanitaryString {
    my( $node ) = @_;
    return 'XML::LibXML'->new( qw/ recover 2 / )
        ->load_xml(string => $node->toString )->toString;
}
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All that is left is to supress the "xmlEscapeEntities : char out of range" warning toSanitaryString generates