keyDown event not working?

local player = game.Players.LocalPlayer
mouse = player:GetMouse()

local damage = 5


function leftpunch()
	local leftpunch = player:findFirstChild("Left Arm")
leftpunch.Position.Vector3.new(30.422, 179.001, 138.099)
leftpunch.Rotation.Vector3.new(90.161, 17.144, -53.438)
wait(1)
leftpunch.Position.Vector3.new(37.5, 179.8, 146.003)
leftpunch.Rotation.Vector3.new(179.669, -0, 180)
end

function keyDown(key)
	if key=="z" then
		leftpunch()
		if leftpunch():onTouched(player) then
			local hum = player:findFirstChild("Humanoid")
			player.Humanoid.Health:TakeDamage(damage)
		end
	end
end

script.Parent.mouse:connect(keyDown)

When you press z, you left punch, But it's not working. Why? Can you explain why too?

put it into 1 function

function KeyDown(key)
	key:lower()
	if key == 'z' then
		--watever arm movements here
		leftpunch.Touched:connect(function(hit)
			h = hit.Parent:FindFirstChild('Humandoid')
			if h ~= nil then
				h:TakeDamage(damage)
			elseif hit.Parent.Name == leftpunch.Parent.Name then
				print('Player cannot Punch himself')
			end
		end
	end
end

mouse.KeyDown:connect(KeyDown)

mouse is not an event. You need to do

local mouse = game.Players.LocalPlayer:GetMouse();
mouse.KeyDown:connect(function(key)
	--key code
end)