But $a and $b always have the risk to be lexicals from the outer scope², I wanted to avoid discussing that problem.¹

It's about naming after all...

¹) compare this error

  DB<108> my $a=3; sort {$a <=> $b} @a

Can't use "my $a" in sort comparison at ...
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²) being bound to another package can be solved by investigating the caller, but checking for lexicals involves padwalker.