What confuses me is that, in Prototypes, it is suggested that a function prototyped (&@) behaves like grep.

The answer is simple! IMHO perlsub is wrong at this place and you proved it! 8)

Congratulations we found the second error in this perldoc chapter within two weeks!

Should be noted that CORE::map has no prototype to be copied, it has hidden magic behavior:

print undef==prototype "CORE::map" # prints 1
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IMHO if there was a possibility to express the behavior with prototypes, perl would use it internally!