I recently asked a similar question and the answers there are very good.

In summary you want to put something meaningless but unique into the cookie (ie a long random number). This is then used to look up the session related data in the database:

my $cookie = $cgi->cookie('session_token');
  # $cookie is '9874354683216735468453121845168441';

if ( $cookie && $cookie =~ m/some match/ ) {
  my ($session_id) = $dbh->selectrow_array
    ("select id from sessions where token = ?", undef, $cookie );
}
[download]

It is the actual presence of the cookie that is used as an indicator that the user is logged in, as well as getting a session_id from the database. All this said the only real way to secure the website is to run it over https as otherwise cookies can be intercepted and used in replay attacks.

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