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 $_ is an alias to the list value, so it can be used to
modify the elements of the LIST. While this is useful and supported,
it can cause bizarre results if the elements of LIST are not variables.
Using a regular foreach loop for this purpose would be clearer in
most cases. See also /grep for an array composed of those items of
the original list for which the BLOCK or EXPR evaluates to true.
{ starts both hash references and blocks, so map { ... could be either
the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
ahead for the closing } it has to take a guess at which its dealing with
based what it finds just after the {. Usually it gets it right, but if it
doesn't it won't realize something is wrong until it gets to the } and
encounters the missing (or unexpected) comma. The syntax error will be
reported close to the } but you'll need to change something near the {
such as using a unary + to give perl some help:
%hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
%hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
%hash = map { ("\L$_", 1) } @array # this also works
%hash = map { lc($_), 1 } @array # as does this.
%hash = map +( lc($_), 1 ), @array # this is EXPR and works!
%hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use +{
@hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
and you get list of anonymous hashes each with only 1 entry.