" I WANT those variables, and having exactly their original purpose and value."

In that case your concern should still be that the variables and patched in code behave badly. If you understand the original code well enough to transplant it then you should understand it well enough to put a nice wrapper around it so it doesn't interact badly with the rest of your code. The wrapper ensures you don't have undeclared variables so your original concern is moot. The trick of course is understanding the borrowed code well enough that it can thrive when transplanted.

Just the process of wrapping the transplanted code in a sub and providing a well defined set of parameters and results will drive understanding of what the borrowed code needs and what it does. Strictures of course help that process by pointing out issues early. If you are seriously concerned that the code behave as expected the next level is to write a test suite against the code.