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  my %opts = ( 'config' => 'default.config',
               'other1' => 'default1',
               'other2' => 'default2' );
  %opts = (%opts, process_command_line());
  %opts = (read_config_file($opts{config}), %opts);

  my %defaults = ( 'other1' => 'default1',
                   'other2' => 'default2' );
...
  my $config = $from_cmdline{config} || "default.config";
  my %from_config = read_config_file($config);
  my %opts = (%defaults, %from_config, %from_cmdline);

  my %defaults = ( param1 => 'default1',
                   param2 => 'default2' );
...
  while (my ($param, $value) = each %defaults) {
    $opt{$param} = $value unless defined $opt{$param};
  }

  my %opts = ( param1 => 'default1',
               param2 => 'default2' );
...
             "param1=s" => \$opt{param1},
             "param2=s" => \$opt{param2});
  read_config_file(\%opt, 'default.config') if $use_default_config;