Apologies, Gregor, for my misunderstanding. I had thought that libgetopt-long-descriptive-perl provided both. I have now removed libgetopt-long-descriptive-perl from my system and the problem remains so, of course, you are right.
By the way, I have confirmed that the problem does NOT occur on my jessie system (perlbase 5.20.2-3+deb8u7). On that system the option parsing works according to the documentation.
