my %opts = ( param1 => 'default1',
               param2 => 'default2' );
  my $use_default_config = 1;
  GetOptions("config|c=s" => sub {
                 my ($param, $value) = @_;
                 $use_default_config = 0;
                 read_config_file(\%opt, $value);
             },
             "param1=s" => \$opt{param1},
             "param2=s" => \$opt{param2});
  read_config_file(\%opt, 'default.config') if $use_default_config;