Ah says he. Logically you just delete the use SelfLoader and __DATA__ tag. On testing of loadtimes using a version of CGI::Simple without Selfloader and the __DATA__ tag the entire script loads/compiles slower but is still faster than CGI.pm plus all the methods are compiled and ready to go.
There would seem to be three options. A mod perl version without the __DATA__ tag. Cutting the module into two parts with the less used functions in another module that gets required in a runtime. A CGI.pm type solution.
I would favour having a CGI::Simple::mod_perl module that is just the standard module without SelfLoader and the __DATA__ tag as this is easiest to maintain and as most scripts need some tuning to use mod_perl so modifying the use CGI::blah should be no big deal.
In reality under mod_perl there is no good reason not to just use CGI.pm as the load time, size, etc is not an issue and it is well proven.