OK now that I can read my own question, the answer seems clear ...

the parser can't decide at compile-time if a scalar like $a holds a coderef, but doing the idempotent operation \& (that means reference of the dereference) makes it undoubtable for the parser.

UPDATE:

prototypes are a compile-time not run-time issue, which explains the confusion.

@JadeNB: defining your own prototype opens the oportunity to say sub f([&$]) { ... } such that at compiletime any scalar is acceptable, which can be assured at runtime to be a coderef!ยน

Cheers Rolf

UPDATE:

(1) Unfortunately this is wrong, there is no sub f([&$]) { ... } for an alternative first argument type, only sub f(\[&$]) { ... }. But this will NOT work like map() or grep(), because now the argument "absolutely must start with that character" (here & or $)

In reply to Re: coderefs and (&) prototypes by LanX
in thread coderefs and (&) prototypes by LanX

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