Quoting perldoc -f sort:

If the subroutine's prototype is ($$), the elements to be compared are passed by reference in @_, as for a normal subroutine. This is slower than unprototyped subroutines, where the elements to be compared are passed into the subroutine as the package global variables $a and $b (see example below). Note that in the latter case, it is usually counter-productive to declare $a and $b as lexicals.

Perl only complains when you try to doubly declare a lexical variable name in the same scope. The following won't produce a complaint:

use strict;
use warnings;

my $foo;

{ my $foo; }
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