$d = "123456789";
$d =~ s/(?<=\d)(?=(\d\d)+$)/-/g;
print  "$d";

##</code><code>##

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

##</code><code>##

 
Variable length lookbehind not implemented in regex; marked by <-- HERE in m/(?<=^(\d\d)+)(?=\d) <-- HERE / at Noname1.pl line 4.