use strict;
use warnings;

my $text = "C:\\letters.txt";

my @letters;
open(IN, $text) || die "Can't open $text";
@letters = <IN>;
close(IN);
chomp @letters;

foreach my $indiv_note (@letters) {
  my $letter_text = ($indiv_note =~ /(^\[Sidenote (?: (?!^\[Sidenote). )* )/);
  print "<div>$letter_text</div>\n";
}

##</code><code>##

[Sidenote: The same.]

                                                     _Sunday Evening._

       *       *       *       *       *

I have at this moment got Pickwick and his friends on the Rochester
coach, and they are going on swimmingly, in company with a very
different character from any I have yet described, who I flatter myself

[Sidenote: Miss Hogarth.]

                            FURNIVAL'S INN, _Wednesday Evening, 1835._

MY DEAREST KATE,

The House is up; but I am very sorry to say that I must stay at home. I
have had a visit from the publishers this morning, and the story cannot