Just remember whether you've encountered "***Private-from-iPhone***" (say in some variable $is_private), and only add the line in this case: 

...
my $is_private = 0;
while (<>) {
  $is_private = 0 if eof;  # reset 
  chomp;
  if (/\Q***Private-from-iPhone***/) {
    $is_private = 1;
  }
  # print additional line
  if ($is_private && /SUMMARY:/) {
    s/$/\nX-CALENDARSERVER-ACCESS:CONFIDENTIAL^M/;
  }
  print "$_\n";
}
...

[download]

(This assumes that DESCRIPTION: occurs before SUMMARY:, of course)

Update: added # reset line, as per GrandFather's/moritz's note — (I had overlooked that the script is supposed to process multiple files in one go).

In reply to Re: help with conditional match? by almut
in thread help with conditional match? by ignatz99

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.