local variables in Perl and Lua

I didnt get an answer for this in the Python thread, so I'll ask here: Why do variables have to have the local keyword to make their scope local? Why cant it just use implicit lexical scope like most any other languages?

Let's pretend Lua had Python's scoping rules. What is happening here?

function()
  a = 0
  return function(x)
    a = x
    return a
  end
end

Where does a live? is the one in the function body a new one or is it assigning to the old one?