... *how* I should go about passing data that I might want to be "private" to a package.

Create your variable privately then don't pass it outside of your package. Likely you want to use my to create the variables.

I am confident that you do not want to be using function prototypes on any of these routines. That is you want: sub function {# do stuff instead of  sub function($) { #do stuff

Function prototypes are a completely different beast in Perl from those in C or C++. You very rarely want them. To emulate the control that C-style prototypes give you you explicitly check @_.

# off-hand code
sub some {
     croak "Bad arg count to some" unless @_ == 2;
     croak "First arg to some not a ref" unless ref $_[0];
     # etc. etc.
     return do_stuff();
}
[download]

Good perling.