##########################################
# Reading
$Afile = "A.txt";
$SEMAPHORE = $Afile . '.lck';
    open(S, ">$SEMAPHORE") or die "$SEMAPHORE: $!";
     flock(S, LOCK_SH) or die "flock() failed for $SEMAPHORE: $!";
     
          open (FH, "$Afile") or die "Can't open $Afile: $!";
            while (<FH>) {
                 ## do_something;
            }
          close FH;

     close S;
##########################################- wrong way
# Writing
$Bfile = "B.txt";
$SEMAPHORE = $Bfile . '.lck'; ### Using $Bfile as $SEMAPHORE file name
    open(S, ">$SEMAPHORE") or die "$SEMAPHORE: $!";
     flock(S, LOCK_EX) or die "flock() failed for $SEMAPHORE: $!";
     
         open (TEMP, ">$Bfile") or die "Can't open $Bfile: $!";
          open (FH, "$Afile") or die "Can't open $Afile: $!";
            while (<FH>) {
                 ## do_something;
            }
          close FH;
         close TEMP;
         rename($Bfile,$Afile);

     close S;
########################################## -right way?
# Writing
$Bfile = "B.txt";
$SEMAPHORE = $Afile . '.lck'; ### Should be using $Afile as $SEMAPHORE file name

etc..