You should check the 5.8.0 man pages "utf", "perlunicode", and probably "perluniintro" as well.

Given that you're migrating older scripts for things that have nothing to do with using unicode data, it should suffice to put:

no utf8;
[download]

near the top of each script (e.g. above "use Date::Manip").

I just added Date::Manip to my 5.8.0 installation and tried a couple of sample operations from the man page, involving the two methods you mentioned. No problem. Is there any chance that your Date::Manip.pm file is actually a layover from an older perl version -- that is, after installing 5.8.0, did you reinstall Date::Manip?

Apart from that, I don't know what to say... maybe if you put up a minimal script that generates the problem for you, and we'll have something more concrete to work with.