"I am fairly happy with it as it is now."

If your changes cover all potential use cases, that's good.

As the original question was focused on POD, I thought I'd just point out a couple of discrepancies.

You wrote "... the parameter $split can be ignored for alpha ..."; but, your POD has "C<split_sort> has four required parameters.". As it stands, users won't know what fourth parameter to use for alpha types; an additional example wouldn't hurt.

Also in that paragraph, the last two sentences are back-to-front: it should be, 3rd is 'type' and 4th is 'expr' (cf. "split_sort($a, $b, 'type', 'expr');" which immediately follows that paragraph).

&split_sort contains no implicit return so the return value will be the result of the last expression evaluated; that will be either a <=> or cmp expression, both of which return one of -1, 0 or 1 (see "perlop: Equality Operators"). The sort itself returns a list which you have correctly assigned to an array. The POD immediately afterwards indicates an ARRAYREF is returned: "returns [ ... ]".

How you deal with that one is up to you. I'd probably show split_sort(...) and indicate it returns the same as <=> and cmp, making it useful for sort. Then show usage with sort much as you currently have. Then show that it returns a LIST instead of an ARRAYREF; just changing [...] to (...) should suffice.

For your 'left' and 'right' examples, I'd mix it up a bit to give a clearer indication of how the sorting process works. For instance, here's a couple of arbitrary examples.

IN:  10:ab 21:bb 2:bb 21:bb 2:b
OUT: 2:b 2:bb 10:ab 21:ab 21:bb

IN:  bb:10 b:1 ab:10 ab:2 bb:9
OUT: ab:2 ab:10 b:1 bb:9 bb:10
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Including the '2' and '02' in your original examples was I good idea. You may want to point out that they'll keep their original order; by which, I mean:

$ perl -E 'say for sort { $a <=> $b } qw{020 2 02 20}'
2
02
020
20

$ perl -E 'say for sort { $a <=> $b } qw{20 02 2 020}'
02
2
20
020
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Finally, as I've not seen the the actual module code, I'm guessing a bit here. The NAME should indicate what the module provides or implements, not what it returns; technically, it should just return a TRUE value — it's common for the last line of code, excluding POD, to be just 1;. Also, it's useful to know whether functions are exported by default or not.