undef EXPR
       undef   Undefines the value of EXPR, which must be an lvalue.  Use only
               on a scalar value, an array (using "@"), a hash (using "%"), a
               subroutine (using "&"), or a typeglob (using "*").  (Saying
               "undef $hash{$key}" will probably not do what you expect on
               most predefined variables or DBM list values, so don't do that;
               see delete.)  Always returns the undefined value.