I think the getkey() function is just an example. In this case, it's a function that somehow returns the right key to put in %hash for the given value. Note the preceding paragraph:

               [...] And

                   %hash = map { getkey($_) => $_ } @array;

               is just a funny way to write

                   %hash = ();
                   foreach $_ (@array) {
                       $hash{getkey($_)} = $_;
                   }
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