### in my code
my $foo = $cgi->param('foo');
# do a bunch of things with $foo, 
# get data out of SQLite,
# prepare html output,
print "Content-Type: text/html\n\n", $template->output;

### in my template
<TMPL_LOOP FOO>
  <TMPL_VAR BAR><br>
</TMPL_LOOP>

##</code><code>##

### in my code
my $foo = $cgi->param('foo');
# figure out if XMLHttpRequest
if ($foo) {
  # do a bunch of things with $foo, 
  # get data out of SQLite,
  # prepare XMLHttpRequest output
  print "Content-Type: text/html\n\n", $output;

# for all other normal cgi requests
} else {
  # grab cgi params,
  # do a bunch of things with them,
  # get data out of SQLite,
  # prepare html output,
  print "Content-Type: text/html\n\n", $template->output;
}

### in my template, 
# document.getElementById("output").innerHTML = output;
<div id="output"></div>