No, my post shows it being used elsewhere. And here's another:

$ perl -le'print __PACKAGE__'
main
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You wouldn't expect @time::ISA to mean @1232553153::ISA. Same goes for __PACKAGE__. It's a built-in function like time.

$ perl -le'print CORE::__PACKAGE__'
main
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Update: I can override it

$ perl -wle'BEGIN { *CORE::GLOBAL::__PACKAGE__ = sub () { "abc" }; } p
+rint __PACKAGE__'
abc
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but I can't call prototype on it

$ perl -wle'print prototype "CORE::__PACKAGE__"'
Can't find an opnumber for "__PACKAGE__" at -e line 1.
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so it's not quite the same as other built-ins. That's probably the only difference.