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  my $template;
  eval {
      $template = new HTML::Template(filename => 'foo.tmpl');
...
  eval {
      print $template->output();
  };

  <TMPL_IF debug>
  <ul>
  <TMPL_LOOP debuglog>
...
  <TMPL_ELSE>
  ... the non-debug page ...
  </TMPL_IF>

  my @debuglog = ();

  if ( ... ) {
...
  ...
  $template->parameter(debug => 0 != @debuglog);
  $template->parameter(debuglog => \@debuglog);

  sub debuglog {
      push @debuglog, {item => $_} for ( @_ );
  }