However, if you disable buffering with $| and print a number of small lines, it will be slower than printing most of a document at once. This, of course, depends on the buffer size of your OS and on such things as packet size. Your web server may also intercept data instead of allowing you to write to a raw socket.

This is one of the last things I would worry about optimizing. You're more likely to get better speed out of using Fast CGI or mod_perl than you are switching from printing a list to printing a here-doc.