Read up on HTTP::Cookies, CGI::Cookie, and hit a few web developer sites. You will find that you can set a cookie for a full domain name or the root domain, you can set the expire time, you can store multiple sets of data in one cookie, and much more.

1 hour, 1 day, 1 week, 1 month, and 1 year are all commonly used depending on how you wish it handled. Some sites set the cookie at one month and bump it up every time it gets under 1 week. That is always nice.

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$you = new YOU;
honk() if $you->love(perl)