$_ is really not what you want. Whenever you'd say "it" in English then use $_, because that's what it is there for. Makes things clean, and it's what I, as the maintainer and reader of your code, expect.

A variable which keeps track of the last variable used has also been mused about in the past. I think it was even implemented in an experiment(this from when I was a member of p5p). The symantics are hard to tie down, and it only leads to obfuscation - it doesn't make one liners clearer, and makes larger programs into a twist of backreferences, all alike.

Light a man a fire, he's warm for a day. Catch a man on fire, and he's warm for the rest of his life. - Terry Pratchet