Notice in the last few examples the use of the local keyword. If you omit the local keyword, the variable is considered to be in global scope. Without the local keyword, the variable is global, no matter what chunk it is in:

foo = 7 -- global
do
    bar = 8 -- global
end
print ("foo: " .. foo)
print ("bar: " .. bar)

The global scope is interesting. It is not tied directly to a Lua file. The local keyword can be used outside any do/end chunks to make a variable local to the file it is loaded from:

foo = 7 -- global, can be accesssed from any loaded lua file
local x = 9 -- local to the .lua file being executed
do
    local bar = 8 -- local to the current do/end chunk
end