You are quite right. However if you have duplicate keys how do you expect the sort to arrange those lines? Do you fall back to a secondary key, or does it not matter, or do you retain the original file order? You could for example rely on sort's stability in recent versions of Perl to retain the lines with identical keys in file order:

use strict;
use warnings;

print sort {substr ($a, 9, 4) cmp substr ($b, 9, 4)} <DATA>

__DATA__
...
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Prints (using the original data):

77809    7211    CA84985    54E
77875    7725    CA84985    54E
77822    7874    CA84985    54E
77873    8003    CA84985    54E
77826    8040    CA84985    54E
77873    8123    CA84985    54E
77819    8503    CA84985    54E
77872    8511    CA84985    54E
77876    8543    CA84985    54E
77884    8543    CA84985    54E
77822    9908    CA84985    54E
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