Update: realized too late that you understood the reason, nevertheless may this answer stand here for others as slow as myself in understanding written english :)

perldoc perlvar :)

$a
       $b      Special package variables when using sort(), see
               "sort" in perlfunc.  Because of this specialness
               $a and $b don't need to be declared (using
               local(), 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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regards,
tomte

Hlade's Law:

If you have a difficult task, give it to a lazy person --
they will find an easier way to do it.

Oh, ha! $a and $b are global variables used by sort and so are already known by perl. That's why you don't see any errors in your example.