The automagic "is or isn't there a sub there?" is giving you grief.

It is trying to treat split as the sort sub, and not as part of how you get a list into the sort.

UPDATE
jeroenes is correct. I should have tested before tossing that out. I guess it is better about finding keywords. But try putting a subroutine that you have created there and it will be cheerfully abused:

sub my_split {
  split "", shift;
}

print join "", sort my_split("qwerty");
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