Can I use rows of copper pennies laid on top of the soil to prevent slugs?
Would the copper pennies leach too much copper into the soil, creating toxicity for our fruits and vegetables?
No, even true copper pennies (those dated before 1982) are not going to decompose fast enough to cause problems. I'm not sure today's pennies will even work, because they're mostly zinc. There are lots of other slug deterrents you can use, though. Read on: https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm