universe of possible hash elements in Perl
+--------------------------------------+
| elements that exist                  |            $hash{element_exists};                  # this example: exists, undefined, false
|  +--------------------------------+  |
|  | elements that are defined      |  |            $hash{element_defined} = function_def();# this example: exists, defined, unknown false/true
|  |  +--------------------------+  |  |
|  |  |  elements that are true  |  |  |            $hash{element_true} = 1;                # this example: exists, defined, true
|  |  +--------------------------+  |  |
|  |                                |  |
|  |  +--------------------------+  |  |
|  |  : elements that are false  :  |  |            $hash{element_false} = function_false();# this example: exists, defined, false
|  |  :  +-------------------+   :  |  |
|  |  :  | false but defined |   :  |  |            $hash{element_false_defined} = 0;       # this example: exists, defined, false
|  |  :  +-------------------+   :  |  |
|  +--:--------------------------:--+  |
|     :                          :     |
|  +--:--------------------------:--+  |
|  |  :  +-------------------+   :  |  |
|  |  :  | false but undef   |   :  |  |            $hash{element_false_undefined} = undef; # this example: exists, undefined, false
|  |  :  +-------------------+   :  |  |
|  |  +--------------------------+  |  |
|  | elements that are undefined    |  |            $hash{element_undefined};               # this example: exists, undefined, false
|  +--------------------------------+  |
+--------------------------------------+