What's there to document? It's a variable name that follows all the rules (letters/digits/underscores, but no leading digits). It's available for you to use because it's not also one with a predefined meaning. Actually, @_ and $_ are likewise free for you to use in any way you choose, although some actions in Perl give them initial values that are interesting.

I would not call it the "default" hash though. There's nothing default about it. {grin}