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    my $len = (-s "file.txt") || 0;
    system ("perl eck.pl $var > file.txt &");
...
        my @newlines = read_new_lines("file.txt", $len);
        $len = $newlen;  # Adjust length to new end of file
    }

    sub read_new_lines {
        my ($fn, $offset) = @_;
...
        chomp(my @lines = <$fh>);
        return (@lines);
    }