-w   prints warnings about identifiers  that  are  mentioned
          only  once,  and  scalar variables that are used before
          being set.  Also warns about redefined subroutines, and
          references  to  undefined  filehandles  or  filehandles
          opened readonly that you are attempting  to  write  on.
          Also  warns you if you use == on values that don't look
          like numbers, and if your subroutines recurse more than
          100 deep.