If you're changing the syntax, why not take the time to fix having to use `local` everywhere instead of making things local by default?
"The problem is that without local declarations, you cannot say where the variable is local to" [2]. Take Python, for example, where "local by default" means "local to the current function". Variables are created in the current function's scope on assignment, which causes 2 problems:
1. It is awkward to modify variables in outer scopes, because an attempt to assign to them creates a new variable. Python works around this with the `global` and `nonlocal` keywords.
2. Python does not support variables which are local to the current block, for example, the body of a loop.
[1] http://lua-users.org/lists/lua-l/2006-10/msg00056.html [2] http://lua-users.org/lists/lua-l/2006-10/msg00063.html
This sort of static checking massively increases the amount of code you can look after without going insane. Add static types too, and you're even better off.
(Last time I used Lua, I modified it so that all variables had to be introduced before their first use, either using `local', or a new `global' keyword. More static checks, and no more accidental globals.)