Uhm, the same way as you do in non-binary files? If the file is small: open the file read/write, read in the file (in a scalar), insert "+" wherever you want it, seek back to the beginning of the file, write the scalar, close the file. If the file is large, open a temporary file for writing, open the original file for reading, read from the file up to the point where you want to insert the "+" while writing what you've read (in chunks) to the temporary file, write "+" to temporary file, read rest of file while writing it to the temporary file, close both files, move temporary file to original file.

Abigail