I guess you are using | as a separator. The problem with your approach is that you want to replace the content, but you are matching the separators as well - but separators are "shared". One way of solving this is the use of split and join:

join "|" => map {length () ? $_ : '\N'} split /[|]/ => $line, -1;
[download]

But that assumes there are no leading or trailing vertical bars. Or you can use lookahead:

$line =~ s'[|](?=[|])'|\N'g;
[download]

Abigail