A major issue with Friend of the Night is that it is always on or off depending on the time of day, meaning in some dark exteriors midday you still can't see, and some well-lit buildings or bunkers will become blindingly bright.

barddarkl made a very good mod that changes the Friend of the Night perk to be toggleable with a hotkey, effectively removing the issue and making it a very helpful perk again. Unfortunately, both me and someone else using it experienced a flickering light effect, meaning that when we turned it off, the game would repeatedly try to turn it back on. This was extremely annoying, so much so that I downloaded the GECK and clumsily made my first mod; a fix for this mod that I really wanted to use.

I have no idea how to mod Fallout. This is a heavy handed, brute force edit that completely erases FotN's script. This means it's never naturally scripted to turn on or off, so the lighting is always at default. Despite this, barddakl's modded in hotkey still enables and disables the effect. I've tested it, and it works for me. Hopefully, this means the flickering lighting effect should be effectively destroyed for anyone else experiencing it, allowing you to enjoy the Toggle Friend of the Night mod without any drawbacks.

I don't have his Perk Overhaul pack, which his toggleable FotN is in apparently, but if it works the same than this should work for that mod as well.