In Lua how do you import modules?

Do you use

require "name"

or

local name = require "name"

Also, do you explicitly declare system modules as local variables? E.g.

local io = require "io"

Please explain your choice.

Programming in Lua 2ed says "if she prefers to use a shorter name for a module, she can set a local name for it" and nothing about local m = require "mod" being faster than require "mod". If there's no difference I'd rather use the cleaner require "mod" declaration and wouldn't bother writing declarations for pre-loaded system modules.