my @old = ( "10.5 AA", "9 AC", "2 BB"); 

my $max_len = 0;
for (@old) { $max_len = length($_) if length($_) > $max_len }

my @new = sort {$b cmp $a} map { 
   my $x=''; 
   $x.='0' for (1..$max_len-length($_));
   $x.$_;
} @old;