s/\|\|/|\\N|/;

##</code><code>##

s/\|(?=\|)/|\\N/g;
s/^\|/\\N|/;
s/\|$/|\\N/;

##</code><code>##

perl -pe"s/\|(?=\|)/|\\N/g; s/^\|/\\N|/; s/\|$/|\\N/" infile >outfile