> The Apple File System, like HFS Plus, found on OS X Macintosh systems is encoded in UTF-8.

It's worse than that. Consider the following program:

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use Unicode::Normalize qw{ NFKD NFKC };

say $^O;

my $letter = "\N{LATIN SMALL LETTER A WITH ACUTE}";

open my $out, '>', NFKD($letter) or die "Open: $!";
unlink NFKC($letter) or warn "Warn unlink: $!";
unlink NFKD($letter) or die "Unlink: $!";
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Running it on Linux and Mac gives the following different outputs:

linux
Warn unlink: No such file or directory at ./script.pl line 13.
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versus

darwin
Unlink: No such file or directory at ./script.pl line 14.
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The same happens when you exchange NFKC and NFKD. Yes, on a Mac, normalization happens on top of UTF-8.