open(FH, "+< filename.txt") || die "$!";  #open the file for read/write
flock(FH, LOCK_EX);        # get an exclusive lock
seek(FH, 0, SEEK_SET);     # move filepointer to beginning of file
read(FH, $scalar, $bytes); # what if I want the whole thing, no matter how many bytes?

# do something with 
# $scalar here

seek(FH, 0, SEEK_SET);     # move filepointer to beginning of file (is this correct syntax for this?)
print FH $scalar;          # print at beginning of file, clobbering old data
truncate(FH, tell(FH));    # truncate any data beyond where we've written (is this correct as well?)
close(FH);                 # close the file