question concerning module architecture

[Geoffrey Leach]

I'm working on updates for Getopt::Auto. In the original, there's this 
block:

END {
   if (exists &main::default) { main::default() }
}

I understand how this works, but, given that its not documented in the 
Getopt::Auto POD, why would main::default() exist?

Secondly, Getopt::Auto looks for main::foo() if the user defines a 
command line option --foo. This is fine, but what if I want to put my 
option code in a different package? Is here any way for a module to 
discover the package that requires it?

Thanks.