That would be... very bad. Don't do it.

The local localizes a global, but doesn't make it thread-local, so you'll be changing thigns for everyone. (And without locking either, which will definitely Do Bad Things)

There's no good way which actually uses %ENV. Best thing to do is to either wrap access in an interface of some sort, or copy %ENV to a different hash.

If you actually need a thread-locak version of %ENV because you're using code you didn't write, well... you're out of luck.