use Fcntl qw(:DEFAULT :flock); 
 
# Open original and temp 
sysopen(FILE, $File, O_RDWR) or die $!; 
  flock(FILE, LOCK_EX); 
  open(TEMP, "$File.tmp") or die $!; 
 
   # Read through and test 
    while(my $Line = <FILE>) 
    { 
      .... is this the nth line? -> go out of the loop 
      print TEMP $Line; 
    } 
     
    # New data and remaining of <FILE> 
    print TEMP $YourNewData; 
    while(my $Line = <FILE>) { print TEMP $Line; } 
  close(TEMP); 
   
  # the re-open should be done better then this. 
  # In fact, be eleminated 
   
  # File copy 
  open(TEMP, "$File.tmp") or die $!; 
    seek(FILE, 0, 0) or die $!; 
    truncate(FILE, 0) or die $!; 
    print FILE <TEMP>; 
  close(TEMP); 
 
# hehe 
close(FILE)