This was very helpful. For my original question, a rejoinder surely could have been "Why don't you just try it and see how it works out?" I did that - I grabbed a test program that I'd built and started modifying it, playing with it, calling it with different args. But the results I was getting made no sense - the paths I put in PERL5LIB weren't showing up in INC.
So eventually I just asked a question.
Yes, the test program I grabbed used taint.
wisdom is slowly dawning
throop