That's not a lot to go on. Please give us more details.

What version of Perl are you moving off of? What version are you moving to?

What version of CGI.pm are you moving off of? What version are you moving to?

Did the move change the network topology from the site's perspective? Specifically, is there now a different proxy server or firewall between the site and customers? (Some firewalls can be configured to block cookies).

From a customer's point of view (out there in DNS-land), did the site's domain name change? That could certainly cause cookie problems, particularly if the site is configured to set cookies under a different domain name that is different from what the browser is responding to.