| is a special character for regexs. It needs to be escaped. 

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

[download]

However, that solution is far from complete.
What if the first field is empty?
What if the last field is empty?
What if there's more than one empty fields?
What if there's more than one empty fields in a row?
Fix: 

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

[download]

As a one-liner: 

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

[download]

In reply to Re: Problem with String replace in file by ikegami
in thread Problem with String replace in file by philosophia

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.