my $pass = cookie('cookie');

# if there was a cookie named 'pass'
if ( defined $pass)
{
    # if the password in the cookie matches the admin password
    if ($pass eq $adminpass)
    {
        # do administrative stuff
    }
}
# there's no cookie, so check for a 'pass' parameter
else
{
    $pass = param( 'pass' );
    if ($pass eq $adminpass)
    {
        # set admin cookie
        # make sure the name is 'pass' so you can retrieve it
        # redirect to the current script
    }
}