map BLOCK LIST
       map EXPR,LIST
               Evaluates the BLOCK or EXPR for each element of
               LIST (locally setting "$_" to each element) and
               returns the list value composed of the results of
               each such evaluation.  In scalar context, returns
               the total number of elements so generated.  Evalu­
               ates BLOCK or EXPR in list context, so each ele­
               ment of LIST may produce zero, one, or more ele­
               ments in the returned value.

                   @chars = map(chr, @nums);

               translates a list of numbers to the corresponding
               characters.  And

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

               is just a funny way to write

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