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   #move this line up (and simplify it):
   #if(length($materialmapping_file) gt 0 or length($data_folder) gt 0
+)
...
   }

   # Now, everything that follows can be outdented one level.

   while( ($filename = readdir(DIR))){
      #if($filename =~ /\.xls$/i) {
...
      next unless defined $oBook;
      # everything below can be outdented
      ...

my %seen;  # a new global variable
...
...
      $materialmapping_item2->setAttribute($entry);
   }