It's documented in perlsub. See the note about "if predeclared" below...

To call subroutines:

    NAME(LIST);       # & is optional with parentheses.
    NAME LIST;       # Parentheses optional if predeclared/imported.
    &NAME(LIST);   # Circumvent prototypes.
    &NAME;       # Makes current @_ visible to called subroutine.
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Probably just an artifact of a parser that uses a single pass. Can't disambiguate something you haven't seen yet.