A Custom Lua module loader for Lua 5.2

loader.lua

-- This allows a more restricted module() style; the key feature
-- is that each module is loaded in a custom environment, and the actual module
-- table is a copy of that environment after initial load.

-- clone _G so that globals from the program don't invade the module
local lua_libs = {}
for k,v in pairs(package.loaded._G) do
  lua_libs[k] = v
end

local function loader(name, modpath)
 local env = {}
 env.module = function (name)
   env._NAME = name
   return env
 end
 setmetatable(env, {
    __index = lua_libs, -- resolve known globals
 })
 local fh = assert(io.open(modpath, 'rb'))
 local source = fh:read'*a'
 fh:close()
 local ret = assert(load(source, modpath, 'bt', env))(name)
 if ret then return ret end -- explicit table was returned
 local mod = {}
 -- the module is a copy of the environment after initial loading
 for k,v in pairs(env) do
    mod[k] = v
 end
 return mod
end

-- replace Lua loader
package.searchers[2] = function (name)
 local modpath, msg = package.searchpath(name, package.path)
 if modpath then
   return loader, modpath
 else
   return nil, msg
 end
end

I think you have a mistake; a searcher returns a string message as its sole return value when it doesn't find anything, so line 39 should be "return msg".