my @new = sort {(split /\s+/, $b)[0] <=> (split /\s+/, $a)[0]} @old; 

##</code><code>##

use Data::Dumper;
use strict;
use warnings;

my @old = ("10.5 AA", "10.6 AA", "9 AC", "2 BB");  
my $t0 = time();
for (1..200000) {#yours
	my @new = map { $_->[0] }
          sort { $b->[1] <=> $a->[1] }
          map { [ $_, (split( /\s+/, $_, 2 ))[0] ] }
          @old;
}
print time() - $t0, "\n";

$t0 = time();
for (1..200000) {#mine
	my @new = sort {(split /\s+/, $b)[0] <=> (split /\s+/, $a)[0]} @old; 
}
print time() - $t0, "\n";