I think we could help you best if you would supply both the relevant parts of the Perl script and the actually errors you receive (the one where you use the heredoc), and the HTML page you are trying to paste.

If you'd rather keep it as a separate page, you should be able to do something like this:

# do all your email stuff
open $fh, "<", "downloads.html" or die;
while ($fh) {
   print;
}
close $fh;
[download]

Or you could slurp in the file and print it all at once, but either way it should work.