If you'd read the CGI::Application docs:

The prerun_mode() method is an accessor/mutator which can be used within your cgiapp_prerun() method to change the run mode Note: The prerun_mode() method may ONLY be called in the context of a cgiapp_prerun() method. Your application will die() if you call prerun_mode() elsewhere, such as in setup() or a run mode method.

I wish to change from one run_mode to another, so that won't work.