If you want a single database (dbm) record to contain "name, email, comments, postime", and you want the dbm file to contain more than one record, you'll probably want to use "email" as the hash/dbm key, and then combine the other three strings into a single scalar string -- something like this:

$BOOK{$email} = join "\t", $name, $comments, $postime;
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Presumably, two different people will not enter different names, comments, etc, using the same email address -- if they do, the one who comes in later will overwrite the one who came in earlier.

This way, when you get the keys from the dbm file, it will be the list of email addresses for people who have signed in; for each key, you can then fetch the name, comments, etc.