1) It doesn't work. (The module exports WeekOfYear, but the sub is called weekOfYear.)
2) You can't get the Week number for an arbitrary date, only today's date.
3) The module doesn't do ISO week of year. ISO's not the only way to do it (Microsoft uses a different algorithm) but the documentation should specify.
DateTime might look like overkill if that's all you're trying to do, but is definitely the way to go.