The only nice thing about zero-width assertions is that you don't have to capture anything. So, in that spirit, and in the spirit of TMTOWTDI, and in the spirit that this is what you were asking about (whether it's the best way to do it or not):
$ perl -e '$x=12345; $x=~s/(?<=\G\d{2})/\012/g; print $x,$/'
12
34
5
As you can see - I just added the \G to the assertion. Of course, if you start using anything other than numbers in there, I'm sure this could get all mucked up:
$ perl -e '$x="123d45"; $x=~s/(?<=\G\d{2})/\012/g; print $x,$/'
12
3d45
