It is free for the using, if not taking. The local keyword is there to facilitate multiple code chunks using the same global variable independently. It works well enough for $_ and @_, it will work just as well for %_. (That's not to say it's a panacea, though.)
I don't understand why people think it's asking for trouble. It is no more asking for trouble than using $_ — and I sure wouldn't counsel anyone to avoid using $_. Quite the opposite, in fact.
That being said, there isn't any really good reason to use %_. The situations in which it would be the most obvious, "natural" name for a variable must be rare indeed.