Hi,
I tried to optimize the substr solution a little.
Now it outperforms join unpack about 70%!

['substr faster'  => sub {
   my $new = $olddate;
   substr($new, 0, 4, substr($new,0,4)."-");
   substr($new, 0, 7, substr($new,0,7)."-");
   $new
}],
[download]

Greetings
MP