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";
}