exists EXPR
Returns TRUE if the specified hash key exists in its hash array, even
if the corresponding value is undefined.
print "Exists\n" if exists $array{$key};
print "Defined\n" if defined $array{$key};
print "True\n" if $array{$key};
A hash element can only be TRUE if it's defined, and defined if
it exists, but the reverse doesn't necessarily hold true.
Note that the EXPR can be arbitrarily complicated as long as the final
operation is a hash key lookup:
if (exists $ref™>[$x][$y]{$key}) { ... }
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