As a corollary, if "abc" is placed before "\K", the substitution appends instead of replaces. 
# Appends to the 3rd occurence of 'abc' from start-of-line
($a = $ref) =~ s/^(?:.*?abc\K){3}/___/;
print $a;

# Appends to the 4th occurence of 'abc' from start-of-line
($a = $ref) =~ s/^(?:.*?abc\K){4}/___/;
print $a;

# omit ^ to append to every 2nd occurence of 'abc'
($a = $ref) =~ s/(?:.*?abc\K){2}/___/g;
print $a;

# omit ^ to append to every 1st occurence of 'abc'
($a = $ref) =~ s/(?:.*?abc\K){1}/___/g;
print $a;

[download]

Of course, the "{1}" is optional in the last pattern.

In reply to Re: Search for <n>th occurrence of regex by ikegami
in thread Search for <n>th occurrence of regex by repellent

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.