Algorithm::Loops produces the fastest and least memory-intensive code out there for what it does. Handling permutations and loops nested to arbitrary depth are the jems of the module, and both allow for iterator usage. The documentation is very complete. The the Filter, MapCar*, and NextPermute functions are easy to use. NestedLoops seems rather difficult to apply because it solves a few big problems in a different way than many programmers think of them. The difficulty is not knowing how or when to use the function, despite the many examples in the POD.
The slowest of the date modules, but one of the most powerful. Includes support for recurring events and custom calendars. Excellent parsing of English dates and times such as '5PM next friday'. It is also pure Perl, making it easier to install on some platforms than similar modules. If you haven't switched to DateTime, and don't need the speed of Date::Calc, Date::Manip is a powerful and easy to use alternative.
This module is great! Not only does it nicely format code, it does an excellent job of telling you where your syntax errors are. If you've ever spent 5 minutes looking for that unmatched brace, bracket or parenthesis--you needed Perl-Tidy.