undef(EXPR)

     undef EXPR

     undef   Undefines the  value  of  EXPR,  which  must  be  an
             lvalue.   Use  only  on  a  scalar  value, an entire
             array, or a subroutine name (using &).  (Undef  will
             probably  not  do what you expect on most predefined
             variables or dbm array values.)  Always returns  the
             undefined  value.   You  can omit the EXPR, in which
             case nothing is undefined,  but  you  still  get  an
             undefined value that you could, for instance, return
             from a subroutine.  Examples:

                  undef $foo;
                  undef $bar{'blurfl'};
                  undef @ary;
                  undef %assoc;
                  undef &mysub;
                  return (wantarray ? () : undef) if $they_blew_it;