What is the requirement for perpetual motion, if the second law no longer applies? The only requirement for perpetual motion is that everything associated with its motion does not produce any heat that can/will be radiated into its surroundings thus degrading the energy of motion!
Induced friction is the primary reason that perpetual motion cannot happen. Induced friction being the generation of heat between matter that is in physical contact, and in relative motion to each other! If we consider perpetual motion to involve the physical movement of a rigid body, then sources of induced friction include:
1) The rubbing of two rigid surfaces (both hard and soft). This includes things like the external friction between a car's tire and the road, or the dragging of an object against the ground. It also includes any internal friction such as heat generated between moving parts within the moving rigid body.
2) The friction of moving rigid body with its non-rigid surroundings. This includes friction commonly known as drag, such as frictional heat generated between a boat and the surrounding water, or a car and its surrounding air.
The above are obvious as they involve relative motion. What may not be so obvious is heat generated in the production of the forces that move the object in the first place. Let us say that the object was powered by a useful system, which does work onto the surrounding atmosphere, i.e. lost work as defined by W=PdV.
As so often described in this website lost work is irreversible because it involves thermal energy i.e. heat or potential energy, being irreversibly given into our atmosphere. This obviously explains why things like the steam engine, combustion driven devices (gas cars) etc. can never be perpetual motion machine.
Do we then add lost work to the above list? Well that really depends upon what we want the list to include. What if a useful expanding system initialized the first impulse but after that there is a cycle that maybe perpetual like the swinging of a pendulum.
Certainly a swinging clock’s pendulum is a near perpetual motion device but is still it experiences friction of the kind described by both 1) and 2). That being: 1) friction between the moving parts. And 2) drag that being motion induced friction with the surrounding air.
Thinking about it, if we could make a device that was both internally and externally frictionless then we may have the makings of a perpetual motion machine. However: Realizing that collisions, even on the molecular scale, are inelastic may limit true perpetual motion to motion inside of a vacuum, whether that be outer space or in a lab.
The above assumes that none figures out how to control heat generated by frictional known as drag or if you prefer viscous dissipation. It should be stated that heat generated by frictions tends to radiate outwards from its source thus dispersing in all directions. Herein, control means collecting this generated naturally dispersing heat, and then funneling it so that it can then be used to further power the device. This has NOTHING to do with either entropy or the second law! It is just a fact of life hotter systems tend to radiate their heat in all possible directions relative to colder systems.
It should be stated that there are those who claim to have created perpetual motion devices but more often than not they seem to be slight of hand gimmicks. However, there are those who claim that our understanding of certain aspects of the science is perverted, much in the way I claim that traditional thermodynamics is. If that is actually the case then all bets are off, yet it still seems strange to even imagine that the first law of thermodyynamics is not valid. If that is really the case then my belief would lean more to our understanding of inertial mass, or Newtonian mechanics is skewed.