I am pretty sure you need to put in the LockDataSource info as well. Below is a snippet from how I use Apache::Session ( I'm sorry but I am not familiar with Mason ).

$dbtype = 'mysql';
$dbname = 'somedb';
$dbname = 'localhost';
$dbuser = 'username';
$dbpass = 'password';
tie %session, 'Apache::Session::MySQL', $id, {
    DataSource    => "DBI:$dbtype:$dbname:$dbhost",
    UserName    => $dbuser,
    Password    => $dbpass,
    LockDataSource  => "DBI:$dbtype:$dbname:$dbhost",
    LockUserName    => $dbuser,
    LockPassword    => $dbpass,
} or die "Can't tie the session ", %session, ": $!";
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EEjack