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    package  Data_holder;
    
...
            .   "I don't know how to set '$key' to '$value'."
        );
    }

    sub set_normal {
        my ($O, $key, $val) = @_;
...
        }
        return ($result);
    }

    sub new {
        {...}
...
        }
        return ($result);
    }

    $O->$func ($key);

    package Command;
    
...
        
        {...}
    }