What do you feed a baby Pennsylvania black racer snake?

Always make sure they have a safe heat source, a cool spot, a place to go hide, and plenty of clean water.
Now, on to feeding... It depends on just how young it is. Start with mealworms when they are just hatched, supplementing with crickets as it grows. Teaching your baby to eat from your hand can be useful in feeding it small pieces of raw meat... stick with lean meats; chopped up rat is a good one. As they grow you can feed them live pinkies, as baby mice are called. You can buy mineral drops to add to their dinner.
Having a pet shop nearby is a necessity, unless you are lucky enough to be able to breed your own mice. Feed it as much as it wants once a week, or several small meals. If it doesn't eat in captivity within three weeks of capture, release it. Nature has designed it to be able to find crickets, spiders and all sorts of grubs and **never** release it during the winter. Most captive kept snakes will go into a type of shock. The temperature difference is too abrupt and what should have been a natural progression of storing body fat in preparation for hibernation... well, that never happened, and they are not properly prepared for it. Keeping a captive snake until late spring is the best time for release. And BTW, Most States don't allow for the native snake population to be depleted by making capturing them anything close to legal. Finding an injured snake and keeping it healthy while it heals is one thing... Taking a baby out of it's environment for no reason other than it is cute... That's just not a great idea.
But if it *is* hurt and won't eat within the three week period, don't despair, you can force feed it. Use a blunt ended pair of tweezers and gently pry the baby's mouth open from the side by lifting up on the top 'lip' making sure you don't injure it's teeth. Place the small chunk of wet meat or mealworm down it's throat just a bit and slowly draw the tweezers out. Ground turkey and chicken are also reasonable substitutes until you can get to the pet shop for those mealworms. You can gauge how big the piece of meat should be by keeping it as small around as the measurement between the snake's eyes. No Way is that gonna be too big. Force feed as much 'stuff' as equals twice the size of the baby's head, wait two days and see if it will eat a live mealworm. If it won't eat in a week, repeat this process. Sooner or later it WILL eat on it's own. One tip, if you are feeding it mealworms, pick them up with the tweezers... some snakes won't eat anything that smells like 'human'.
I found a foot-long black king snake on the side of the road with a three inch gash down it's side where it had been run over by a car. He was eating by himself within a month. He went back to the wild ten months later with nothing more than scars down his side. I still see him out in my garden from time to time... that was two years ago and he is three feet long now.
Good luck with your baby.