An easy (and educational) way to get answers to questions like this is to consult the source.

vars.pm is quite short, consisting essentially of a single 30-line import routine, of which ~10 lines are interesting. Even if parts of the code use uncommon (though hardly non-standard) trickery, it is quite easy to answer the specific question of whether repeated applications of use vars are cummulative.

Source is your friend. It will answer all sorts of questions if only you take the time to ask it.