package MyXMLSimple;
use base 'XML::Simple';

sub sorted_keys { my ( $self, $name, $hashref ) = @_; return sort { main::element_order($a) <=> main::element_order($b) } keys(%{$hashref}); };

... SNIP ...

return $parser->XMLout($xml,
        KeyAttr       => [],
        RootName      => 'Zymonic',
        NoEscape      => 1,
        SuppressEmpty => 1
);