It's discussed in Apocalypse 6, though lore had it that $_ would indeed be lexically scoped to each sub.

Furthermore, a method will see $_ as "this" upon entry, so it will almost never work the old way (sorry, Abigail-II).

Page 9 shows "Your caller's current topic is named $CALLER::_." and goes on to illustrate how to use it as a default parameter, so if you call it without an argument the function uses the caller's $_. But the function can call it by whatever formal name it likes for that parameter.