map is a looping function to apply an expression or a block to every item in a list (each item in turn assigned to $_), and collect/return the list of results. It's like: 
my @results;
foreach(@_) {
    push @results, &$CODE();
}
return @results;

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where $CODE is the expression (delimited by a comma) or block (no delimiter) in the first argument.

example: 

@output = map $_*2, 1 .. 10;

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which is identical in result to 
@output = map { $_*2 } 1 .. 10;

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and which return the even numbers from 2 to 20; the double of all numbers between 1 and 10.

In reply to Re^5: hash array by bart
in thread hash array by Anonymous Monk

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