The easiest way to do it using Date::Manip would be to grab the documentation and use something very like example #4 ("The amount of time between two dates.") to determine the difference between the two dates, then use something very like example #5 ("To determine a date a given offset from another.") to apply the calculated offset to other dates.

The most efficient way to do it would likely be to not use Date::Manip, as detailed in the "SHOULD I USE DATE::MANIP" section of the documentation. (Although I can't tell you what would be the more efficient module to use because I tend to value "easy" over "efficient" and would just use Date::Manip despite the author's explanation of why I shouldn't.)