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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. Evaluates BLOCK or EXPR in a 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. See also "grep" for an array composed of those items of the original list for which the BLOCK or EXPR evaluates to true.