It may be that CGI::Ajax expects build_html to return the html (or updated html) for the page. However, by default, $tt->process($template, $vars) will print the processed output to STDOUT. Try something like the following:

  sub main_page {
    my $output = '';
    $tt->process($template, $vars, \$output)
       || die $tt->error(), "\n";
    return $output;
  }
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which will stuff the processed template into $output.