Hmm... I will confess that I don't know much about mod_perl or Apache::Session, but I'll speculate a little since no one has replied to this yet.

First, I notice that you are not checking your return codes on your SQL. For example:

$sth->execute($login, $passwd) or die $dbh->errstr;
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If your SQL is not executing, this should give you some clues. Also, are you checking the return codes on your connect and disconnects?

$dbh = DBI->connect($data_source, $user, $password) or die $DBI::errst
+r;
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Or, you can use RaiseError to avoid checking the return value of each call:

$dbh = DBI->connect($data_source, $user, $password, {
             RaiseError => 1});
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Note that failure to connect or disconnect returns undef and sets $DBI::err and $DBI::errstr. Do not bother checking $! as it will not be set. I've never tried to execute SQL against a database that I have not successfully connected to, so I am curious if that may cause problems.

Second, are you properly disconnecting from the database? I believe, like open files, that mod_perl will choke if you forget to close a database handle and then try to reopen it.

Here's another bug that has bitten me: I have written code that connects or disconnects from databases based upon conditionals. As a result, I have sometimes had (shame on me) a close get skipped because I wrote my conditionals poorly.

Are you locking the table and forgetting to unlock it? Perhaps your code is waiting for a table to unlock? And are you accessing locked and unlocked tables at the same time? Here an annoying little snippet from the book "MySQL & mSQL":

Using locked and unlocked tables at the same time can cause the process thread to freeze. You must lock all of the tables you will be accessing during the time of the lock. Tables you access only before or after the lock do not need to be locked.

Incidentally, newer versions of MySQL do not have this problem.

I know most of this is probably irrelevant, but hopefully there may be a useful nugget or two.

Cheers,
Ovid