my $len = (-s "file.txt") || 0;
    system ("perl eck.pl $var > file.txt &");

    # ... later ....
    my $newlen = (-s "file.txt") || 0;

    if ($newlen > $len) {
        my @newlines = read_new_lines("file.txt", $len);
        $len = $newlen;  # Adjust length to new end of file
    }