trying to understand the perldoc description givees me too much of a headache:-

A hash or array element can be true only if it's defined and defined only if it exists, but the reverse doesn't necessarily hold true.

here's a Venn(ish) diagram in beautiful ASCII art that may or may not help, with examples on the side

universe of possible hash elements in Perl
+--------------------------------------+
| elements that exist                  |            $hash{element_exis
+ts};                  # this example: exists, undefined, false
|  +--------------------------------+  |
|  | elements that are defined      |  |            $hash{element_defi
+ned} = 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_fals
+e} = function_false();# this example: exists, defined, false
|  |  :  +-------------------+   :  |  |
|  |  :  | false but defined |   :  |  |            $hash{element_fals
+e_defined} = 0;       # this example: exists, defined, false
|  |  :  +-------------------+   :  |  |
|  +--:--------------------------:--+  |
|     :                          :     |
|  +--:--------------------------:--+  |
|  |  :  +-------------------+   :  |  |
|  |  :  | false but undef   |   :  |  |            $hash{element_fals
+e_undefined} = undef; # this example: exists, undefined, false
|  |  :  +-------------------+   :  |  |
|  |  +--------------------------+  |  |
|  | elements that are undefined    |  |            $hash{element_unde
+fined};               # this example: exists, undefined, false
|  +--------------------------------+  |
+--------------------------------------+
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Note that the ASCII art combined with wanting space for labels sometimes implies there is room in the Perl universe for combinations that aren't actually possible: for example, there are no elements that are undefined but not false, because perl coerces undefined to false.