It may very well be gone someday in perl5. Really. Don't use it. Also note that the staticness is relative to the pad, not the function; for instance, the data does not persist for recursive calls to the function.
Re^4: my $cache = undef if undef;
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If you look a few nodes further up the thread you'll see I'm advising against its use anyway. :-) I'm simply saying that there is a lot of inertia against its disappearance. That's not a promise, of course. It does work in the meantime though, and can very occasionally be useful so long as we're talking about throwaway code (I used it in a oneliner once to save a few keystrokes.)