Thanks for your response -- I posted this problem last thing at night, then went to bed. When I woke up this morning, my mind returned to this problem, and I thought, "That's ridiculous -- could it be a mis-print?"

  DB<6> @w3 = ( $foo =~ /(?=(\d{3}))/g );

  DB<7> x @w3
0  123
1  234
2  345
3  456
4  567
5  679
6  790
[download]

Yep. A ':' where it should have been a '='. Guess I better check to see what other errata there are.