sub kennethk {
    my $string = shift;
    my $off = 0;
    my $step = 10;
    my @indices = sort {$off = 0;
                        until (substr($string, $a+$off, $step) cmp substr($string, $b+$off, $step)){
                            $off += $step;
                            $step *= 2;
                        }                                                                       } 0 .. length($string) - 1;
    $_++ for @indices;
    return @indices;
}

##</code><code>##

RobertCraven 2.90/s
kennethk     12.8/s
xxx          13.1/s
hdb          14.2/s