From http://stein.cshl.org/WWW/software/CGI/:

Several other named parameters are recognized. Here's a contrived example that uses them all:

   print $query->header(-type=>'image/gif',
                        -status=>'402 Payment Required',
                        -expires=>'+3d',
                        -cookie=>$my_cookie,
                        -charset=>'UTF-7',
                        -Cost=>'$0.02');
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-expires

Some browsers, such as Internet Explorer, cache the output of CGI scripts. Others, such as Netscape Navigator do not. This leads to annoying and inconsistent behavior when going from one browser to another. You can force the behavior to be consistent by using the -expires parameter. When you specify an absolute or relative expiration interval with this parameter, browsers and proxy servers will cache the script's output until the indicated expiration date. The following forms are all valid for the -expires field:

        +30s                              30 seconds from now
        +10m                              ten minutes from now
        +1h                               one hour from now
        -1d                               yesterday (i.e. "ASAP!")
        now                               immediately
        +3M                               in three months
        +10y                              in ten years time
        Thu, 25-Apr-1999 00:40:33 GMT     at the indicated time & date
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When you use -expires, the script also generates a correct time stamp for the generated document to ensure that your clock and the browser's clock agree. This allows you to create documents that are reliably cached for short periods of time.

CGI::expires() is the static function call used internally that turns relative time intervals into HTTP dates. You can call it directly if you wish.