my %defaults = ( param1 => 'default1',
                   param2 => 'default2' );
  my $config = 'default.config';
  GetOptions("config|c=s" => \$config,
             "param1=s" => \$opt{param1},
             "param2=s" => \$opt{param2});
  %defaults = (%defaults, read_config_file($config));
  while (my ($param, $value) = each %defaults) {
    $opt{$param} = $value unless defined $opt{$param};
  }