upvalue_hack.lua

local print = print    -- upvalue index 1
local a = 100          -- upvalue index 2

function func()
    print("in func: ", a, b)
end

-- set global 'a', 'b' to function 'func'
setfenv(func, { a = 200, b = 101 })

func()

-- modify the upvalue 'a' of function 'func'
debug.setupvalue(func, 2, 10001)

func()

-- print the upvalue 'a' of function 'func'
print("upvalue: ", debug.getupvalue(func, 2))

-- print the local 'a' (modified)
print("a: ", a)

--[[
in func: 	100	101
in func: 	10001	101
upvalue: 	a	10001
a: 	10001
]]