Typically browsers don't cache CGI output. You can adjust this by messing with the cache headers sent back from the web server. Are you using CGI.pm? Just use the -expires value in the header method:

print $query->header(-expires=>'+3d');
[download]

That'll set the document to expire in three days. You may want something different. :) Look at the docs for CGI.pm.

You could also look at the HTTP spec for the relevant headers. There's a Cache-Control header, specifically.

Finally, you could also try changing to a GET method if it's not too much trouble.