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Any mechanism that could switch the magnet on and off would require energy. E.g. you could put a magnet next to it with opposite poles, which would mostly cancel out the magnetic field, but would require a lot of force to push them together, which is work, and thus requires energy.

No reason you couldn't do that as long as you just mean the actuation is purely mechanical. You'd still need an electromagnet and power source though, as it only works if you can turn the magnet on and off.

The easiest perpetual motion is just drop a tennis ball. It is just that the perpetually moving thing is no longer the ball :-)

What do you propose to use as an energy source?

Just build a second machine to power the first!

clockwork wind-up. It would be bulky but maybe you could put a permanent magnet on a sled or wheel that would impart the necessary force on the metal ball as it approached but then and flipped repelled it. Would be difficult to make work smoothly or at all...