This is something I've been aware of for a while, but not spent too much time on.
Looking at the perl testers reports, all of the 5.10.0 computers fail with this, but 5.10.1 works fine. I will need to install 5.10.0 to figure out if there's a workaround.
5.10.1 did make several changes and bug fixes related to UTF-8, so I'm a bit hesitant to spend a lot of time on this. On the other hand, Date::Manip has 31 failures out of 986 testers (on perl testers) and all but 2 or 3 of them are due to this, so if I can figure out a workaround, it will certainly make Date::Manip look better.