How do I make the ball follow the character's torso but 2 studs in front of it?

You should probably use something like head or rootpart if you use R15. Regardless you need a simple loop that does multiple times what you already scripted. I will also clean up your syntax and make some suggestions.

--you should refer to the variables you define
player = game.Players.LocalPlayer
char = player.Character or player.CharacterAdded:wait()
hum = char:WaitForChild('Humanoid')
head = char:WaitForChild('Head')
mouse = player:GetMouse()
uis = game:GetService('UserInputService') --mouse KeyDown is deprecated.
keys = Enum.KeyCode

uis.InputBegan:connect(function(input,gpe)
	if not gpe then --not on an interface
		local key = input.KeyCode
		if key == keys.E then
			local fb = game:GetService("ServerStorage").Fireball:Clone()
			game:GetService("Chat"):Chat(head, "Fireball!", "Red")
			fb.Parent = game.Workspace
			fb.Anchored = true
			while wait() do
				fb.CFrame = head.CFrame + Vector3.new(0,0,-2)
			end
		end
	end
end)

player.CharacterAdded:connect(function(character)
	--make sure these variables are updated
	char = character
	hum = char:WaitForChild('Humanoid')
	head = char:WaitForChild('Head')
end)