Yes, $a and $b are special, as mentioned. Think about it, suppose sort would not use special variables, and you could pick any variable you wanted. How would sort than distinguish between:

    sort {$a <=> $b}
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and

    sort {$b <=> $a}
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Or what if you have something like:

    sort {$a < $c ? $b <=> $d : $a <=> $e}
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then what?