The problem here is that, if do_something_else() takes a while, and uses a lot of resources, a will still be hogging up memory while it runs -- Lua's only confident that it can release a after chunk() returns.
The fix is to wrap local a inside it's own block. i.e.:
(Now a can be freed before do_something_else() is called.)
So, while you could certainly hack together a "strict locals" patch, one that would do for locals about what strict.lua does for globals -- I'm not convinced it's a particularly good idea. If the problem we're trying to fix is unnecessary memory use by locals, the solution is to use more do-blocks. Enforcing naming discipline on locals is a mostly unrelated topic -- and most examples of local shadowing will happen inside nested blocks, a situation where they're not, actually, likely to be a source of memory inefficiencies.