Current Perl documentation can be found at perldoc.perl.org.
Here is our local, out-dated (pre-5.6) version:
They may or may not implement the
DEFINED() methods differently. For example, there isn't the concept of undef with hashes that are tied to
DBM* files. This means the true/false tables above will give different results when used on such a hash. It also means that exists and defined do the same thing with a
DBM* file, and what they end up doing is not what they do with ordinary hashes.