Simple module definition that prevents overwriting globals by accident *N.B. This won't work on Lua >= 5.2 because of the new _ENV*

gistfile1.lua

local M = {
	__index = _G,

	__newindex = function( t, k, v ) 
		if rawget( _G, k ) ~= nil then
			print( string.format( "Setting key '%s', on table %s masks a value in _G!!", tostring( k ), tostring( t ) ) )
			error()
		else
			rawset( t, k, v )
		end
	end 
}

setmetatable( M, M ) -- re-use table. Saves having an extra table just to hold __index and __newindex

setfenv( 1, M )

-- table = 5    -- 1) un-comment to cause an error 
-- M.table = 5  -- 2) M.table == table == _G.table 

return M