+1 AC for a perception penalty is a fair trade.
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Investigation is not commonly used, and susceptibility to flanking would add a whole new system. The exhaustion ticking is too easy to counter and difficult to track.

However, perception is extremely important both in and out of combat. Being forced to take a penalty on perception would mean that donning a helm is a choice with advantages and disadvantages. Even during combat it may be better to remove your helm to locate a hiding foe, or listen for approaching foes. 

Having modifiers to abilities isn't common though. Usually 5e relies on disadvantage. You could implement a simple helm that seems balanced like so:

>**Helmet**

>While wearing a helmet, you gain a +1 bonus to AC, but you have disadvantage on Wisdom (Perception) checks that rely on sight or hearing.

However, if you are playing a hack 'n' slash style game that is purely straight up meat-grinder combat where perception is never used, then it's a no-brainer +1 AC.

It is also one more thing you have to equip before a fight. This could have a big impact on action economy and the way that your character behaves out of the fight. If your helm is stowed then it would take free object interaction to pull it out, and an action to don it. If you keep it in your hand, that's one less thing you can hold, and it still takes free object interaction so if you want to draw your weapon on the same turn you will be using an action.