A routine that turns defined $hashref->{$key} into $self->isDefined($hashref, $key) doesn't seem to have any purpose other than obfuscation to me. At least as far as I'm concerned, your code immediately became twice as clear once I substituted that for defined.

Maybe it's an interface consistence thing that makes you have this method in your class - but that doesn't mean your routine should be using it.

Makeshifts last the longest.