Perception

Summary
At this time, unless you hunt, there is no reason to invest in Perception. If you do Hunt, getting to Perception level 4 should be a priority.

Details
Each level of Perception reduces the rate at which your plane's condition drops, and increases the chance of encountering pirates on hunts only. It does not increase the number of combats on risky, safe or normal flight legs. With more Perception, your plane can last longer between services, but the extent of this modification to the rate of plane condition decrease is unknown.

Plane Condition
Perception reduces plane condition decay noticeably, but the exact amount of reduction is not known.

Hunting
As evident from the combat encountered timing (which can easily be seen in the AIM notifications) every 300 seconds (or 5 minutes) the game engine will check to see if it should generate a combat. Perception levels of 3 or less have somewhat low probability of succeeding while a Perception of 4 has a 90-95% chance of generating a combat. A Perception of 5 practically ensures at least one combat opportunity will appear (at least) every 5 minutes of a Hunt.