To answer your original question why don't you just do:

my $cgi = new CGI;
my $must_have_param = $cgi->param('foo');
if(not defined $must_have_param)
{
  $must_have_param = 'Alive!';
}
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In this case your actual script just tests if the form input named 'foo' was submitted, and if not assigns's a default value. If your concerned about using the param interface I'm fairly certain you could just do $cgi->param('foo','Alive') if not defined $cgi->param('foo'); To set the param foo to a certain value inside the cgi object.