You can look at all the stuff that's already in an established session DB with something like this:

use Data::Dumper;
$Data::Dumper::Terse = 1;
$Data::Dumper::Indent = 1;
use DBI;
use Apache::Session::MySQL;

my $dbh = DBI->connect('DBI:mysql:apache_session' .
                       ':' . 'localhost' .
                       ';mysql_read_default_file=/path/to/.my.cnf',
                       undef, undef,
                      );

( my $sth = $dbh->prepare('SELECT id FROM sessions') )->execute;
while ( my $id = $sth->fetchrow_array ) {
    my %session;
    tie %session, 'Apache::Session::MySQL', $id, {
                                                  Handle     => $dbh,
                                                  LockHandle => $dbh
                                                 };
    print Dumper \%session;
# you could edit or do whatever to %session here and get the
# data back with the same $id later
}
$sth->finish;
$dbh->disconnect;
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For creating and manipulating the sessions, you should use the module itself, as CountZero suggested. It is in control of the "a_session" column's data (just a text field) and its serialization/retrieval, and you should almost certainly leave it that way.