One trick you can also use (in addition to simply doing a redirect) is to take advantage of the
multipart/x-mixed-replace MIME type, which is only supported by Netscape. If you're familiar with multipart MIME types, this is essentially identical, except each "part" will replace the previous part in the browser window. This is useful for things like JPEG image streaming. Sadly, IE has never chosen to adopt this functionality.
$|=1;
print <<"EoF";
Content-type: multipart/x-mixed-replace; boundary=1234
--1234
Content-type: text/html
<h1>Test 123...</h1>
--1234
EoF
sleep 2;
print <<"EoF";
Content-type: text/html
<h1>Test 456...</h1>
--1234
EoF