use Data::Dumper;

use strict;
use warnings;

my @old = ( "10.5 AA", "10.6 AA", "9 AC", "2 BB");  
my @new;
{
	no warnings;
	@new = sort {$b <=> $a} @old; 
}
"a" == "a"; #meaingless other than to demo the fact that the warnings is back on
print Dumper \@new;