$a and $b are special vars (you really shouldn't use them). From perlvar:

               Special package variables when using sort(), see
               "sort" in perlfunc.  Because of this specialness
               $a and $b don't need to be declared (using use
               vars, or our()) even when using the "strict
               'vars'" pragma.  Don't lexicalize them with "my
               $a" or "my $b" if you want to be able to use them
               in the sort() comparison block or function.
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