+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.

However, perception is 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.

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.