$d =~ s/(\d\d)(?=\d)/$1-/g;

##</code><code>##

1 while $d =~ s/(?<=\d\d)(?=\d)/-/;

##</code><code>##

$d = reverse($d);
$d =~ s/(?<=\d)(?=(\d\d)+$)/-/g;
$d = reverse($d);