Instead of running perl with the -w switch, use warnings; and turn off the particular warning in a scope. That needs Perl 5.6+.

$ perl -Mwarnings -MData::Dumper -e '
> @old = ( "10.5 AA", "9 AC", "2 BB");  
> @new = do {no warnings q/numeric/; sort {$b <=> $a} @old}; 
> print Dumper \@new;'
$VAR1 = [
          '10.5 AA',
          '9 AC',
          '2 BB'
        ];
$
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(Added) If you're in pre-5.6 perl, or you just want to do it this way, this works:

$ perl -w -MData::Dumper -e '
> @old = ( "10.5 AA", "9 AC", "2 BB");  
> @new = do {local $^W = 0; sort {$b <=> $a} @old}; 
> print Dumper \@new;'
$VAR1 = [
          '10.5 AA',
          '9 AC',
          '2 BB'
        ];
$
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That knocks out all warnings in the scope.