my twig is v.3.14
That's the explanation. You NEED 3.16. First the split on a given level (-l option, or default) will not work, and then merging back will not work because of the way processing instructions were handled (NOT!) when parsing with the twig_roots and twig_print_outside_roots options.
I am afraid you will have to upgrade ;--)
3.16 is tested with perl 5.8.5/expat 1.95.7/XML::Parser 2.34/whatever else is useful on linux, perl 5.8.6/expat 1.95.8/ recent version of other modules on Mac OS 10.3, more tests are on the way.