If a force always acts perpendicular to an object's direction of motion, that force cannot change the object's kinetic energy.

This is very good conceptual question and can clear your doubts regarding work-energy theorem.
Whenever force is perpendicular to the direction of the motion, work done by that force is zero.
According to work-energy theorem,
Work done by all the force = change in kinetic energy.

here, work done = 0.
Therefore, 
0=change in kinetic energy
This means kinetic energy remains constant.
Hope this helps

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