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my %hash = (
    foo   => 'bar', # Exists, Defined, Positive length, True.
...
    print "$key\'s Boolean value is ", ($hash{$key} ? "true.\n" : "fal
+se.\n");
    print "\n";
}

foo exists.
foo is defined.
...
blink has a length of zero because it is not defined.
blink's value is undefined.
blink's Boolean value is false.