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

open my $FH, "the_file";   # open for reading
flock($FH, LOCK_SH);       # lock it
...
push @lines, "new line\n"; # process the lines
print $FH for (@lines);    # write them out
close $FH;                 # close and drop lock

# Open a different file as a semaphore lock
...
close $FH;                 # close the file

close $SEM;                # close and unlock semaphore

# Open the file for locking purposes only
...
close $FH;

close $SEM;                # close and unlock semaphore