I like JavaFan's one-liner solution, provided that the assumptions it makes are suitable to your situation. It assumes that each file isn't too big to fit in memory, that the 10 bytes you want to replace will need to have the same replacement value in all files, and that all files of interest have names that don't begin with ".".
in reply to Changing data with syswrite ?
If the replacement value needs to vary from one file to the next (and assuming you have a way for a perl script to work that out), it would be easiest to use JavaFan's method: for a given file, slurp it into a single scalar (e.g. $_), apply a regex replacement, and write a new copy of the file.
If your files are actually too big (which these days means more than a gigabyte or two per file), you'll want to use the read and write functions (rather than "sysread" and "syswrite"); that way, you'll be using buffered i/o in all operations. Follow JavaFan's advice (and read the man pages, too) about how and when to use "tell" and "seek"; depending on what's supposed to go into those 10 bytes that you're replacing, you might need to use pack.