Because something like my @idx = map /(id\d+)/, @items; is a lot more natural than

my @idx;
/(id\d+)/ && push @idx, $1 for @items;
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(which is already pushing legibility). Basically when you build one list out of another, non-destructively, map is the ticket. It can be in other cases as well, but those can often go either way.

Makeshifts last the longest.