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

exists may also be called on array elements, but its behavior is much 
+less obvious and is strongly tied to the use of delete on arrays.

WARNING: Calling exists on array values is strongly discouraged. The n
+otion of deleting or checking the existence of Perl array elements is
+ not conceptually coherent, and can lead to surprising behavior.

#!/usr/bin/perl
use strict;
...
          undef,
          2
        ];