...

 my $filename = 'C:\letters.txt';
 open my $fh, '<', $filename or die "Can't read $filename, $!";
 my $text = do { local $/; <$fh> };
 close $fh;

 my @sidenotes  = split /(?=\[Sidenote:)/, $text;
 my $div_output = join "\n", map "<div>$_</div>", @sidenotes;
 
 print $div_output;

 ...