Is it actually a problem though. My garage pegboard has a permanent bend but all the wrenches and pliers are still hanging there after years. Unless you're storing entire engine blocks, a little sag never hurt anybody. People online make it sound like your whole wall will collapse. My dad's had the same droopy board since the 90s and it's fine.
Replaced my saggy pegboard with a sheet of half-inch plywood and put French cleats on it like @kellyflores said. The cleats lock onto matching strips on the wall so the whole thing can't really sag forward. It holds my heavy drill and impact driver no problem now.