In the CGI docs (and also the docs for CGI::Cookie), once you get past the part about setting cookies, it tells you how to read cookies.

CGI:

use CGI;
my $query = new CGI;
my $sessionid = $query->cookie('sessionID');
[download]

CGI::Cookie:

use CGI qw/:standard/;
use CGI::Cookie;
my %cookies = fetch CGI::Cookie;
my $sessionid = $cookies{'sessionID'}->value;
[download]