Any response you'll get here is 'IMHO', so the important thing is really readability for you, both today and in a year when you have to maintain this code.

From my perspective, using a compound assignment operator improves the legibility dramatically. With the large number of hash keys present, it'd be very easy to get confused whether the keys are all the same. += makes your intent much more obvious. It also protects you from potential typos.

Make sure in migrating this code that you are actually reading the code properly. It be a real shame to put time and energy into improving the readability and break the code in the process.