G'day Lady Aleena,

"I just can not come up with a list where this might be needed on alphabetical strings."

With split_sort(), as currently written, I can't see any use for the alpha sort type: it will return the same as a plain sort (in all cases, as far as I can tell).

With an alpha sort type ignoring case, you could get this difference:

$ perl -E 'my @x = ("ade:Y", "Abc:X", "Afg:Z"); say for sort @x'
Abc:X
Afg:Z
ade:Y

$ perl -E 'my @x = ("ade:Y", "Abc:X", "Afg:Z"); say for sort { fc($a) 
+cmp fc($b) } @x'
Abc:X
ade:Y
Afg:Z
[download]

With an alpha sort type expecting Unicode, you could get this difference:

$ perl -C -E 'my @x = ("\x{c5}de:Y", "Abc:X", "Afg:Z"); say for sort @
+x'
Abc:X
Afg:Z
Åde:Y

$ perl -MUnicode::Collate -C -E 'my @x = ("\x{c5}de:Y", "Abc:X", "Afg:
+Z"); say for Unicode::Collate->new->sort(@x)'
Abc:X
Åde:Y
Afg:Z
[download]

"I am now thinking that I should just put a note in saying that the alphabetical usage is redundant."

Perhaps not entirely redundant. Consider its potential use in a scenario where you process an AoA which holds a mixture of numeric and alphabetic arrays.

my @multi_sorts = (
    [ $array1, ':', 'num' ],
    [ $array2, '-', 'alpha' ],
    [ $array3, ',', 'num' ],
);

handle_multi_mixed_sorts(\@multi_sorts);

# At this point in the code, each of the arrays in @multi_sorts
# has the original array still as the first element
# and the sorted array now as the fourth element.

sub handle_multi_mixed_sorts {
    my ($multi_sorts) = @_;

    for my $i (0 .. $#$multi_sorts) {
        push @{$multi_sorts->[$i]}, [
            sort { split_sort($a, $b, 
                    $multi_sorts->[$i][1], $multi_sorts->[$i][2])
            } @{$multi_sorts->[$i][0]}
        ];
    }

    return;
}
[download]

Do note that I just typed that code directly into my post: it's entirely untested.