use strict;
use warnings;

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

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

foreach my $indiv_note (@letters) {
    if ($indiv_note =~ /\[Sidenote\:\s(.*?)\]/) {
        print "<div>$1</div>\n";
    }
}