Most perks in Fallout 4 are just supposed to give you a little boost, nothing major or almost game-breaking. This is not one of those perks.
Whenever you befriend a companion in Fallout 4 and max your relationship status with them, they will grant you a special Perk. Piper, for example, will give you "The Gift of Gab," which allows you to gain double XP when you do speech checks with other people, or when you discover new places.
One character in Fallout 4, MacCready, can give you a perk known as "Killshot." Officially, it's just supposed to give you a 20% boosted chance to hit any enemy on the head, which makes sense given that MacCready himself is a sniper.
Killshot is a pretty significant Perk. 20% is huge! For whatever reason, though, the Perk is glitched out so that it grants the player a perpetual 95% chance to nail headshots instead. It's an absurd glitch:
Notice how far away I am from the enemy there. Still 95%. I tested this out from the maximum distance that VATs will trigger on, and it still always gave me 95%.
Even if the enemy was partially obscured:
And even if every single other body part had a zero per cent chance of being hit — I still got 95%
If you'd like nab this perk, you have to find MacCready of course. MacCready is waiting for you inside of The Third Rail, a location within Goodneighbor. You can find MacCready in the back room of the bar, and he will ask you for a few hundred caps to hire him, depending on where your Charisma is at.
Once you have MacCready, you have to appeal to his senses. Unlike most companions in Fallout 4, MacCready is more of a troublemaker. He likes it when you steal, and when you pressure people into giving you more caps for a job. In general, he likes it when you're a dick. If you get him to like you enough, he will eventually give you a specific personal quest. To complete it, you must go inside of Med-Tek and retrieve an item inside, which you can only do once MacCready gives you a password to enter that area of the locale:
Once you do everything for this quest, you still have to push MacCready's affinity to "Idolizing" you. This took a while of travelling for me, since I don't really role-play as a dick in Fallout 4, but you should be able to do it after a few hours of travelling with MacCready.
Do note that having this Perk may completely change the game for you, so nab it at your own risk. Bethesda may eventually patch it. For now, though, if ridiculous headshots in Fallout 4 is what you're after, MacCready is your man. Just know that having a 95% chance to hit doesn't necessarily mean you'll hit hard — you still need a good weapon/damage/appropriate levelling for that. Don't go running after legendary Deathclaws unprepared just because you have this Perk!
And, if you're a player who is actually sick of this Perk and its brokenness, good news! PC players can actually download a mod to fix the 95% glitch, and you can find that here.