wouldn't it immediately change again as soon as the callback exits, anyway, as the foreach goes into its next iteration?

It would, but it would also clobber the original entry from @callbacks. Here's a demonstration:

my @callbacks = (
  sub { print "one\n" },
  sub { print "two\n"; $_ = "three"; }
);

print "@callbacks\n";
for (@callbacks) { $_->() }
print "@callbacks\n";
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