Apparently, there's ambiguity in the parser that does not get resolved as desired.

a b->c()

desired: a( b->c() )
actual:  ( b->a() )->c()
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unary-plus is a no-op, but it changes what the compiler sees from a bareword to an operator. That removes the ambiguity.

Using parens around the parameters (ok( ... )) would also remove the ambiguity.

- ikegami