It doesn't have to. Under Unix time keeping, leap seconds don't exist. Computer clocks will have to be adjusted.

While the the whole POSIX time / leap seconds issue is a rather complicated mess (for those interested in gory details see The Future of Leap Seconds, History of IEEE P1003.1 POSIX time and Unix time) I'm not sure how it's relevant to how Date::Calc handles leap seconds?

Indeed, as documented, Date::Calc says that Time_to_Date() and Date_to_Time() take/return UTC time values that include leap seconds. The documentation appears to be lying though :-)