Most wise monks:

I'm having some difficulties. I want to open a file for read/write and immediately obtain an exclusive lock, read the whole thing into a scalar (or array), do something with the data I read, then write it all back to the file (clobbering whatever was once there), then close it. But how do I do all this? I thought I saw a good explanation in a post a week ago, but I couldn't find the post. And I also tried reading the manpages, but was unable to decipher all the functions. Perhaps I'm missing something.
Here's what I've got:

open(FH, "+< filename.txt") || die "$!";  #open the file for read/writ
+e
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 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
[download]

So the problem is that this seems to leave me with an empty file, rather than reading and then writing what I had. Perhaps I'm intermixing some seek/read/truncate calls incorrectly or something. In addition, if I simply want to read the entire file into the scalar using read(), how would I do that, since I don't know the length (in bytes) to ask for? Honestly, I did read some man pages for read, seek, truncate, tell, but I must be missing something, or making a blatantly obvious mistake.

Any help you can provide is much appreciated.
Thanks! --Kozz