playermanager.lua

--[[hook to "lib/managers/playermanager"
you should to replace the names of the following:

(line 25)
"yuri_customperkdeck_register_listeners"
^ should be a better unique hook id of your choice

(line 26)
self:has_category_upgrade("player", "placeholder_health_regen_upgrade")
^ "player" should be the category of your upgrade; "placeholder_health_regen_upgrade" should be the same name of the upgrade id you made in upgradestweakdata.
This upgrade should be a true/false value, which indicates whether or not the player has the ability to heal upon a headshot kill.

(line 30)
self:get_upgrade_value("player","yuri_health_regen_amount",0)
^ "player" should be the category of your upgrade; "placeholder_health_regen_upgrade" should be the same name of the upgrade id you made in upgradestweakdata.
This upgrade should be a number value (inside a table), in case you want to make this amount upgradeable in later card tiers of your custom perk deck

(lines 27 and 37)
"activate_yuri_health_regen"
^ should be a unique "event id", or the name of what should happen when you get a headshot kill. don't worry too much about it, just make it unique like the hook id.

--]]


Hooks:PostHook(PlayerManager,"check_skills","yuri_customperkdeck_register_listeners",function(self)
    if self:has_category_upgrade("player", "placeholder_health_regen_upgrade") then
        self._message_system:register(Message.OnLethalHeadShot, "activate_yuri_health_regen", function()
            local player = self:local_player()
            if alive(player) then
                local restore_hp_amount = self:get_upgrade_value("player","yuri_health_regen_amount",0)
                if restore_hp_amount ~= 0 then
                    player:character_damage():change_health(restore_hp_amount)
                end
            end
        end)
    else
        self._message_system:unregister(Message.OnLethalHeadShot, "activate_yuri_health_regen")
    end
end)