Because the assignment copies the values, this also has the effect of
       turning call-by-reference into call-by-value.  Otherwise a function is
       free to do in-place modifications of @_ and change its caller's values.

           upcase_in($v1, $v2);  # this changes $v1 and $v2
           sub upcase_in {
               for (@_) { tr/a-z/A-Z/ }
           }