Is a very interesting problem but difficult to experiment with.. anyway you can try to use twig_roots or you can try to preprocess your input.
In fact I see a declared lenght in the header: will be possible to read only what is declared in Content-Length and pass this chunk to XML::Twig to be processed.

Maybe you can elaborate a specific XML::Twig question as new SOPW, the author of the module lurks here sometimes..
L*

UPDATE: you can also read this interesting article