... add ("\0" x $n) to the original string.

Shouldn't that be ("\0" x ($n-1)) if $n is the boundary width in bytes?

Win8 Strawberry 5.8.9.5 (32)  Sat 08/20/2022 15:46:20
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

use bytes;

use Data::Dump qw(pp);

use constant BYTE_BOUND => 4;
use constant PAD => "\0" x (BYTE_BOUND-1);

my $s = 'TheRainInSpainFalls';

while (length $s) {
    my $p = $s . PAD;
    $p = bytes::substr $p, 0, bytes::length($p) - bytes::length($p)%BY
+TE_BOUND;
    printf "%4s - %-28s \n", bytes::length($p), pp($p);
    chop $s;
    }

^Z
  20 - "TheRainInSpainFalls\0"
  20 - "TheRainInSpainFall\0\0"
  20 - "TheRainInSpainFal\0\0\0"
  16 - "TheRainInSpainFa"
  16 - "TheRainInSpainF\0"
  16 - "TheRainInSpain\0\0"
  16 - "TheRainInSpai\0\0\0"
  12 - "TheRainInSpa"
  12 - "TheRainInSp\0"
  12 - "TheRainInS\0\0"
  12 - "TheRainIn\0\0\0"
   8 - "TheRainI"
   8 - "TheRain\0"
   8 - "TheRai\0\0"
   8 - "TheRa\0\0\0"
   4 - "TheR"
   4 - "The\0"
   4 - "Th\0\0"
   4 - "T\0\0\0"
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