From @pjscott

  Can't use subscript on %s
  (F) The compiler tried to interpret a bracketed
  expression as a subscript. But to the left of the
  brackets was an expression that didn't look like an
  array reference, or anything else subscriptable.

Wouldn't that be better off as "... array or hash reference, or ...",
since it's just as likely to be triggered by a spurious {} as a spurious [] ?