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

  eval {
      $template->param(param1 => $somevalue);
      $template->param(param2 => $someothervalue);
  };
  print $@ if $@;

  eval {
      print $template->output();
  };
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The first eval catches typos in the template. The second eval catches both mismatches in parameter names (e.g., "payrate" vs. "pay_rate"). I forget now what the third eval catches, but it did catch something odd for me once, so I recommend it. Doing this cut my debug cycle in half.

Turning Templates into Error Logs

Instead of (or in addition to) dumping diagnostic information into a separate logfile, it's possible to turn a template into an error log, via the following technique:

In the template, switch between the error log and "normal" by embedding the following:

  <TMPL_IF debug>
  <ul>
  <TMPL_LOOP debuglog>
    <li><TMPL_VAR item ESCAPE=HTML>
  </TMPL_LOOP>
  </ul>
  <TMPL_ELSE>
  ... the non-debug page ...
  </TMPL_IF>
[download]

Use this by doing

  my @debuglog = ();

  if ( ... ) {
      push @debuglog, {item => "Some message"};
  }
  ...
  $template->parameter(debug => 0 != @debuglog);
  $template->parameter(debuglog => \@debuglog);
[download]

For convenience you can do something like

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

so that you can write   debuglog("Some message") if ( ... ); The beauty of this approach is that you get a debug log if there are problems, or a normal page if there aren't. Debug cycles can be wickedly quick.

I adapted this scheme to report "normal" errors, and use it to give users a punch list of things to fix, like "Please provide a valid credit card number".