The easiest way to think of map is to reduce it to something a bit more familiar.

@result = map SOME_EXPRESSION, @input;
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can be replaced with:

@TEMP = ();
foreach $_ (@input) {
  push @TEMP, SOME_EXPRESSION;
}
@result = @TEMP;
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In other words, evaluate SOME_EXPRESSION for each @input item, and take the result(s) of that, concatenated together, to make an output. While the expression is being evaluated, $_ contains the "current item" from @input.

-- Randal L. Schwartz, Perl hacker