open my $FH, ">the_file"; # zero-byte the file
flock($FH, LOCK_EX);      # and Then lock it...

##</code><code>##

open my $FH, "the_file";   # open for reading
flock($FH, LOCK_SH);       # lock it
my @lines = <$FH>;         # read it
open $FH, ">the_file";     # open for writing: LOCK DROPPED.
# Now someone else picks up the lock and finds an empty
# file.  When they're finished,
flock($FH, LOCK_EX);       # lock it again
push @lines, "new line\n"; # process the lines
print $FH for (@lines);    # write them out
close $FH;                 # close and drop lock

##</code><code>##


# Open a different file as a semaphore lock
open my $SEM, ">the_file.semaphore";
flock($SEM, LOCK_EX);       # lock it

# then process the Real File
open my $FH, "the_file";
my @lines = <$FH>;         # read it
# because we never fiddle with $SEM, we still have a lock
open $FH, ">the_file";     # open for writing.
push @lines, "new line\n"; # process the lines
print $FH for (@lines);    # write them out
close $FH;                 # close the file

close $SEM;                # close and unlock semaphore

##</code><code>##


# Open the file for locking purposes only
open my $SEM, "the_file";
flock($SEM, LOCK_EX);       # lock it

# Process it
open my $FH, "the_file";
my @lines = <$FH>;
open $FH, ">the_file";
push @lines, "new line\n";
print $FH for (@lines);
close $FH;

close $SEM;                # close and unlock semaphore