Brushed DC motors can be biased to spin in one direction faster than the other(Either due to design or discrepancies in manufacturing) referred to as motor timing. On some motors, the difference is inconsequential, but on others, they can spin noticeably faster in one direction than the other to effect performance drastically, to the point that the manufacturer might recommend one direction over the other. The arrow on the PERM refers to what its timed for(CCW). You can reverse the motor direction like you would with a regular brushed DC motor, it just won't spin as fast. You can alter the motor timing by messing with the brushes, but unless you're an experienced electrical engineer and have a strong notion of what you're doing, I don't recommend it.
The differences in timing are only noticeable in two wheeled bots, which are more vulnerable to minor wheel speed changes. Four wheeled bots and onward are a lot less vulnerable to motor differences.
Also, why are you looking at the PERM? Its a terrible motor for combat.
