You can replace map with a for loop and an assignment. There is an example of allocation to a hash with both map and a loop to be found in the map documentation http://perldoc.perl.org/functions/map.html

From Perldoc

Map always returns a list, which can be assigned to a hash such that the elements become key/value pairs. See perldata for more details.

  
    %hash = map { get_a_key_for($_) => $_ } @array;
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is just a funny way to write

    %hash = ();
    foreach (@array) {
        $hash{get_a_key_for($_)} = $_;
    }
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Cheers,
R.