Did you, by any chance, install a pre-packaged module (versus one installed from source by the cpan shell or similar)?

Try to reinstall the module. If cpan install Date::Manip says it's current and it still doesn't work (had something similar a few times with ActivePerl), try this to see whats going on:

cpan
look Date::Manip
perl Makefile.PL
make test 

make install 
# Might have to use sudo make install
# (depends where your perl modules are installed)

make distclean
exit
exit
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