The text of the tutorial doesn't mention the fact that the scope of the effect of use vars is file-level. This is, however, documented in the perldoc for vars:
the "use vars" and "use subs" declarations are not BLOCK-scoped. They are thus effective for the entire file in which they appear.(it's also mentioned under "Pragmatic Modules" in the perlmodlib man page).
Even though its "maximum reach" is one file, the effects of use vars are still different than those of our, which was all I was trying to document in the tutorial.
So, the general answer to your question is that you can't do exactly what you say you want to do, and that's one of the points of use strict: if you're going to use a variable in a file, you have to declare it explicitly in that file; it encourages you to know where everything's coming from, and protects you from typos in variable names.
This doesn't mean your problem's insoluble, though. If, you want inc.pl to serve the function of being a "configuration script" that initializes a bunch of values so other scripts can use them, you can have it work this way: create one data structure (say, a hash), and 'export' that hash. This way, a script that uses inc.pl will only need to declare one variable with our or use vars; of course, with a hash, you lose the safety mentioned as one of the chief benefits of use strict.
For this reason, instead of using a simple hash, you might want to have inc.pl be an object oriented module, but that's beyond the scope of this tutorial.
If not P, what? Q maybe?