while( ($filename = readdir(DIR))){
      #if($filename =~ /\.xls$/i) {
      next unless $filename =~ /\.xls$/i;
      # everything below can be outdented	
	
      my $parser     = Spreadsheet::ParseExcel->new();
      my $oBook  = $parser->parse($data_folder.$filename);
      #if (defined $oBook ) {
      next unless defined $oBook;
      # everything below can be outdented
      ...