use Apache::Cookie;
use Apache::Session::File;

# Inside your mod_perl handler

my $cookie = Apache::Cookie->new($r,
    -name    =>  'session_id',
    -expires =>  '+1D',
    -domain  =>  'mysite.com',
    -path    =>  '/'
);

my %session;
my $session_id = $cookie->value;

# The tie will fail if the session_id isn't valid or is missing.  Thus the need for eval.
eval {
    tie(%session,'Apache::Session::File',$session_id, { Directory => '/tmp', LockDirectory => '/tmp' });
};
if ($@) {
    # bogus session_id.  Setting it to undef will cause Apache::Session to create a new one
    undef $session_id;
    tie(%session,'Apache::Session::File',$session_id, { Directory => '/tmp', LockDirectory => '/tmp') || die "say something useful";
}

if ($session_id != $cookie->value) {
    # session id has changed or is a new one; update the cookie
    $cookie->value($session_id);
    $cookie->bake;
}

# use %session at will to store user data.

$session{'name'} = 'John Doe';
$session{'user_id'} = 123;

my $value = $session{'some_data'};

# untie the session after you're done with it
untie %session;