abc.log
12
15
17

##</code><code>##

write_log("12345");

sub write_log {
    my ($message) = @_;

	my $new_data = '';
	my $header_fields_value = 0;
	my $length = length $message;
	open (MYFILE, '>>', 'abc.log');
	$header_fields_value = read_line(3);
	$new_data = $header_fields_value+$length+2;
	$header_fields_value =~ s/$header_fields_value/$new_data/g;
	seek(MYFILE,3,0);
	print MYFILE $header_fields_value;

	close (MYFILE);
}

sub read_line {
    my ($offset) = @_;
    open (MYFILE1, '<', $filename); 
    seek(MYFILE1,$offset,0);
    my $line = <MYFILE1>;
    close (MYFILE1);
    return($line);
}