$config->{$section}->{$option} = $defaults->{$section}->{$option}
            unless $config->{$section}->{$option};
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I would throw the word exists (or defined) in there after the unless, just in case a user ever wants to specify a false value (e.g., a blank subject line is very rarely a good idea, but occasionally when sending mail to certain kinds of really old autoresponders that read commands from the subject line it can be desirable; there are also highly specialised cases for a blank From field).