Note that, because $_ is a reference into the list
               value, it can be used to modify the elements of
               the array.  While this is useful and supported, it
               can cause bizarre results if the LIST is not a
               named array.  Similarly, grep returns aliases into
               the original list, much as a for loop's index
               variable aliases the list elements.  That is,
               modifying an element of a list returned by grep
              (for example, in a foreach, map() or another
               grep()) actually modifies the element in the
               original list.  This is usually something to be
               avoided when writing clear code.