The documentation [...] didn't quite shed any light on it,

I thought misunderstood the question and have already struck out that part of my post while you were composing your reply.

But why isn't the my that I'm using here doing exactly that??

For loop iterators are aliases for efficiency and to allow you to modify the list.

for my $s (@strings) {
   $s = trim($s);
}
[download]

But I'm still curious why [foreach my $letter ('a' ... 'c')] Doesn't exhibit the same behavior?

There's no range operator in that statement. for (X .. Y) is a counting loop. It doesn't flatten X .. Y. See Re: For vs. Foreach