Given a date/time as an array of (year, month, day, hour, minute, second), how would you convert it to epoch time, i.e., the number of seconds since 1970-01-01 00:00:00 GMT?
Possibly one of the better examples of 'There's More Than One Way To Do It", with or without the help of CPAN.
If you have control over what you get passed as a 'date/time', I'd suggest going the DateTime route, either by using a specific Date::Time::Format subclass, or using DateTime::Format::Strptime if there isn't one supporting your wacky date format (see the datetime FAQ for more details). In general, Date::Time is the way to go if you want to do anything serious with the result: few classes on CPAN are quite as anal-retentive and obsessively accurate.
If you're expecting weird freeform stuff, throw it at Date::Parse's str2time() method, which'll get you a seconds-since-epoch value you can then have your wicked way with, without the overhead of Date::Manip.