Just to clarify. Parens immediately after a function are taken as delimiters for the argument list (as you rightly explained) --- but that has nothing to do with Perl's prototypes:

sub foo { # no prototype here
    return ('a','b','c');
}
# print (foo)[0]; # same error
print +(foo)[0];
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I also show the unary + op as a better generalization of your last example --- it disambiguates without imposing numeric context.