I notice your use of the indirect object form at the very end to avoid parens around (just) brackets. But didn't do so in the other place, because it's not at the end. Moving one paren doesn't make it much better. But you could use feed notation and lose the parens in both places:

$result= (  $x.split('') ==> map { %rtoa{$_.uc} } ==> reduce { $^a+$^b
+-$a%$b*2 }  );
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but you still wind up with one paren after a brace. Or if you reversed the written order, so "everything to the right" is indeed the argument to the listop, I think this works:

reduce {$^a+$^b-$a%$b*2 } map { %rtoa{$_.uc} } $x.split('');
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which makes me think that there is merit in doing it that way as originally presented. listops are naturally written with the processing sequence in the opposite order. Fighting it means adding parens, no matter how you try.