use Benchmark 'cmpthese';

my $sref = {
    neglook => sub {
        my $string = "abababa";
        $string =~ s/(b)(?!.*b)/x/;
        return;
    },
    rindex => sub {
        my $string = "abababa";
        my $ri  = rindex( $string, 'b' );
        substr( $string, $ri, 1) =  'x';
        return;
    },
    sexeger => sub {
        my $string = "abababa";
        $string = reverse $string;
        $string =~ s/b/x/;
        $string = reverse $string;
        return;
    },
};

cmpthese( 3_000_000, $sref );

__END__

perl5.8.4
            Rate neglook  rindex sexeger
neglook 523195/s      --    -21%    -36%
rindex  659776/s     26%      --    -19%
sexeger 813670/s     56%     23%      --


perl5.6.2
            Rate neglook  rindex sexeger
neglook 719252/s      --    -19%    -20%
rindex  888889/s     24%      --     -1%
sexeger 901442/s     25%      1%      --