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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. Evaluates BLOCK or EXPR in list context, so each element of LIST may produce zero, one, or more elements 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) {
    $hash{getkey($_)} = $_;

Note that, because $_ is a reference into the list value, it can be used to modify the elements of the array. While this is useful and supported, it can cause bizarre results if the LIST is not a named array. Using a regular foreach loop for this purpose would be clearer in most cases. See also "grep" for an array composed of those items of the original list for which the BLOCK or EXPR evaluates to true.