#!/usr/bin/perl

use strict;
use warnings;
use Fcntl qw(:flock);

$| = 1;    # Flush the output

my $file    = "perl.txt";

open( FH , "+<" , $file )
    or die "Unable to open '".$file."' - $!\n";
flock( FH , LOCK_EX )
     or die "Could not lock '".$file."' - $!\n";

=notes
    +< open a file for updating without truncating it.
    (truncating means empties the file before opening it.
    flock exclusive lock or FH , 2 but better LOCK_EX
    because, might not be the right number on all
    operating systems.
=cut

seek( FH , 0 , SEEK_END )
    or die "Cannot seek - $!\n";
# put the pointer at the end of file for writing.
    
truncate( FH , 0 )
   or die "Cannot truncate - $!\n";

print FH "This is line-1."\n";
print FH "This is line-2."\n";

close(FH) or die "Could not write '".$file."' - $!\n";