Perpetual motion machines
Take a UV-grade fused silica vessel with 1 cm walls, annealing point 1140 C. Heat it to 900 C under hard vacuum both sides (turbopump plus titanium sublimation pump) with electrodeless low pressure 184.9 nm Hg vapor UV flood to dissociate everything not silica. Let that proceed for a few days, then flame off the connection. Cool. Cool in superfluid liquid helium. There's your "perfect" vacuum. It still has zero-point energy (Casimir effect) plus thermal emission.
Here's a "perfect" perpetual motion machine of the Second Kind. Why is it crap?
A hermetically isolated hard vacuum envelope contains two closely spaced but not touching, in-register and parallel, electrically conductive plates having micro-spiked inner surfaces. They are connected with a wire, perhaps containing a dissipative load (small motor). One plate has a large vacuum work function material inner surface (e.g., osmium at 5.93 eV). The other plate has a small vacuum work function material inner surface (e.g., n-doped diamond "carbon nitride" at 0.1 eV). Above 0 kelvin, spontaneous cold cathode emission runs the closed isolated system. Emitted electrons continuously fall down the 5.8 volt potential gradient. Evaporation from carbon nitride cools that plate. Accelerated collision onto osmium warms that plate. Round and round. The plates never come into thermal equilibrium when electrically shorted. The motor runs forever.