From my pov I would not write the following things in perl:
- Complex multi-threaded programs (because objects/structures can not be shared and function pointers can not be used cross-threads) as well. Library support for simple multi-threaded things like async IO is not good. It is also not acceptable that it is unclear if a CPAN module supports multi-threading or not.
- Complex GUIs - Tk has a old-fashioned look-and-feel. Other GUI kits need to improve on stability and documentation. The thread issue is even bigger with complex GUIs as you need to pass around objects between the two things.
- Shared objects - nearly impossible to roll out perl as a COM object or a shared library for other platforms. It is also very painful to acess shared objects.