"Perpetual motion" is perhaps a flaws term. It doesn't actually mean "constant motion". It refers to the fact that you can't transfer energy indefinitely (transfer energy, not maintain it). If you had a disc spinning in a vacuum, there'd be no transference of energy, the energy-state of that disc will remain the same.
On the other hand, if you had a ball bouncing off two walls (think pong in space), energy is transfered from the ball, to the wall, back to the ball, then to the other wall, etc. Energy is continually being transfered but it cannot go on forever. Free energy is lost with every interaction and entropy increases.
To understand why this is fundamentally so, you'll have to consider how work is done. Work is done through a difference of energy states (a high energy state transfers energy to a low-energy state in the attempt to reach an equilibrium). In order for energy to be perpetually transferred, all of the energy from the high energy state would have to be transferred to the low energy state.
This
picture should demonstrate what I'm talking about. After the reaction (transference of energy from high to low state) both states remain idle and none of the energy can be used (because there is no more high or low state). Now, you can either use the remaining energy and find another energy state that's lower (since with each reaction, the total energy state will become more even, it'll become harder and harder to find an energy state that is lower) or you can artificially create a higher energy state (such as lighting up gasoline) so that it can react and produce work.
Either way, with each transference of energy (action-reaction), all the energy states of the universe become more and more even. As it becomes more and more even, less and less work can be done (work relies on high energy states and low energy states, if they're all medium-energy states, no highs and lows, no work can be done). This is what perpetual motion means. That energy transfers (from high to low state) cannot continue indefinitely, each transfer "evens out" the energy states, afterwards, there is no high and low state anymore and hence, no work can be done.
In order for an energy transfer to end up with just as much useful energy as before (that is, there is another high energy state just as high as before and a low energy state just as low as before), you'd have to have a 100% transfer of energy from one place to another. That doesn't happen, not naturally. It would require a 0-level energy state (that is, a place with absolutely no energy). As far as we observe, there is no such place (microwave background radiation appears to exist everywhere).
Now, here's the disclaimer. In order for the above to be true, energy cannot be created nor destroyed. If you could just create energy, you can just generate higher and higher energy states. In recent (not sure just how recent, maybe 10+ years already) experiments, at a very small scale (at the subatomic level), it appears that indeed energy is created and destroyed all the time. It's created in very miniscule amounts (so small that they weren't detected before) and then quickly vanishes again. This was predicted by physicists before it was actually observed (at least, I think that's it). The Uncertainty Principle states that the momentum and position of a particle (which is just a bending of space-time) can be known or predicted. That is, it's not defined. Well, if any point in spacetime was at a certain, definite energy level (say 1 Kelvin), then it is defined and known. If a point in spacetime had no energy (0-level), then that would be known and defined. And so, it was hypothesized that there is an underlying flux (Zero-level energy I think it's called) in which the energy state constantly (and, as far as we can tell, non-deterministically) shifts. With no observable reaction from the outside (no energy being transfered to it), particles (such as an electron) would suddenly gain enough energy to move and break through certain barriers. "Quantum effects" as its sometimes refered to.
If this effect could be harnessed (and in some ways, it can, such as the Casimir effect), it could potentially produce true perpetual motion devices and prevent the universe from dying. After all, if usable energy is constantly diminishing, there will reach a point where there will be no more usable energy in the universe. All energy-states will be the same, no more highs or lows. The universe will reach a state of thermal equilibrium (all points being the same) and no work will be done (and hence, no life). Depressing isn't it?