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open(FH, "+< filename.txt") || die "$!";  #open the file for read/writ
+e
flock(FH, LOCK_EX);        # get an exclusive lock
...
print FH $scalar;          # print at beginning of file, clobbering ol
+d data
truncate(FH, tell(FH));    # truncate any data beyond where we've writ
+ten (is this correct as well?)
close(FH);                 # close the file