...
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";
}
...