`map', `grep' and other list operators are intended to transform arrays    into other arrays by applying code to the array elements one by one. For    speed, the elements are referenced via a `$_' alias rather than copying    them. As a consequence, if the code block of the `map' or `grep' modify    `$_' in any way, then it is actually modifying the source array. This IS    technically allowed, but those side effects can be quite surprising,    especially when the array being passed is `@_' or perhaps    `values(%ENV)'! Instead authors should restrict in-place array    modification to `for(@array) { ... }' constructs instead, or use    `List::MoreUtils::apply()'.