As for the question you added,

Explain why you think this……should produce different output than this…

I agree that there should be a function that does that, but it's not reverse.

Explain why you think

my $water_samples = join '', map chr, 113, 101, 769;
my $last_sample = substr($water_samples, -1, 1);
print(ord($last_sample), "\n");
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should produce different output than

my $water_samples = join '', map chr, 113, 101, 769;
$water_samples = reverse($water_samples);
my $last_sample = substr($water_samples, 0, 1);
print(ord($last_sample), "\n");
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Update: Corrected my numbers.