I've (again) restarted the Apache server. mod_perl currently throws errors for SQL statements that otherwise run fine, so maybe some memory corruption happens for long-running processes...

DBD::mysql::st execute failed: You have an error in your SQL syntax; c
+heck the manual that corresponds to your MySQL server version for the
+ right syntax to use near ' NOW() )
[download]

for

    my $in= $DB->getDatabaseHandle->prepare(
        'REPLACE INTO iplog( user_id, ip_id, tstamp )'
      . ' VALUES( ?, ?, NOW() )'
    );
    $in->execute( getId($USER), $ip );
[download]

Update: (and looking at the mysql access log, it is even right in its error message (one IP address redacted, the other, blank space is as-is)):

               1566594 Query       REPLACE INTO iplog( user_id, ip_id,
+ tstamp ) VALUES( '5348', 1234567890, NOW() )
               1566350 Query       REPLACE INTO iplog( user_id, ip_id,
+ tstamp ) VALUES( '806480', , NOW() )
[download]

As to who/what circumstances allow DBD::mysql (3.0008) to create this invalid SQL for the placeholder-guarded statement is quite unclear to me.