Your setup starts with a certain amount of potential energy: There is an attractive force between the cars, but the cars are not touching.
Car B is not free to roll toward car A (ratchet wheels), so the only motion that is possible when you release Car A is, car A rolls (accelerates) toward car B. The initial potential energy is coverted to the kinetic energy of car A.
Then they collide, inelastically, and they roll off together at some velocity that can be computed by applying the laws of conservation of momentum and conservation of kinetic energy.
And that's the end of the story. In a universe with no friction, they will keep rolling forever, with no change in momentum and, no change in kinetic energy.
In our actual practial universe, there will be friction between the wheels and the road and, in the wheel bearings. The kinetic energy will be converted to heat, the cars will slow to an eventual stop, and the heat will dissipate, increasing the entropy of the universe.
Either way, all of the energy that we're talking about is the energy that you put into the system in the first place by separating the cars from each other, working against the attractive force of the magnet.
No new energy will be added or generated after you release car A.
The only thing wrong with your thinking is the idea that perpetual motion is impossible. It's not. The proof is in the heavens (e.g., like how the Earth has been circling the Sun for billions of years.)
What is impossible is the so-called perpetual motion machine (a.k.a., over-unity device). That would be a machine that not only runs forever, but it also would produce enough new energy to compensate for energy lost to friction, and perhaps also to do some kind of useful work. Nobody has ever built one yet, and nobody who's learned their physics lessons is likely to ever invest in one.
