exists function
Test whether a hash key is present
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 be TRUE only 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 $ref->{"A"}{"B"}{$key}) { ... }
Although the last element will not spring into existence just because its existence was tested, intervening
ones will. Thus $ref->{"A"} $ref->{"B"} will spring into existence
due to the existence test for a $key element. This autovivification may be fixed in a later release.
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