use Benchmark;

my $string = "1234567890";

timethese(-5, {
                'Regexp'      => sub { $string =~ s/(.*?)(....)$/("X" x length($1)).$2/e; },
                'Regexp D&P'  => sub { $string =~ s/.(?!.{0,3}$)/x/g; },
                'Substring'   => sub { $string = length substr($string,0,-4). substr($string,-4) }
              });