I normally create a hash with default values, then map param('name') to hash{'name'}. The default may even be '' but this way I get no warnings.

Another way to handle this is to assign to the param, like

$q->param('subject') || 
  $q->param(-name=>'subject', -value=>'no subject');
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warning: 0 or '' is still false, so the default will step in
Tiago