I opened the files I had to access (read/write) with ">>" and it worked as I expected.

 ">>" mode is not read/write.   From perlopentut:

       To open a file for appending, creating one if necessary:

           open(FH, ">> $path");
           sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT);
[download]

Notice the O_WRONLY (write only) attribute.