use strict;
use warnings;

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

open(my $fh_in, '<', $text)
   or die("Can't open $text: $!\n");

my $open = 0;

while (<$fh_in>) {
   if (/^\[Sidenote /) {
      print("</div>\n") if $open;
      print("<div>\n");
      $open = 1;
   }
   print;
}

print("</div>\n") if $open;