There are a number of problems:

1. At the moment you use tell, you get the position the file pointer is: at the end of the current line. The regexp match does not influence that.
2. seek takes three arguments, a file pointer (FH), a position to seek to, and an argument which determines how that position should be interpreted: from the beginning of the file, from the end, or from the current position. Your arguments don't make sense.
3. A sysread advances the filepointer as well. If you want to replace the 10 characters you read, you got to seek 10 positions back before doing your syswrite.
4. Your $BUF is empty -- shouldn't that contain at least 10 characters?
5. As the documentation of sysread says: sysread uses unbuffered I/O, don't mix it with buffered. Do not use sysread when you also use <>.
6. You are trying to open (as a file) anything in the directory, including . and ...

Perhaps the following works for you (untested):

$ perl -0777 -pi -e 's/(SCSI:INQ:80.{94}).{10}/${1}TEST123456/sg' /hom
+e/bkirch/PERL_LSN/TICKETS/*
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