I'm of the opinion that references should be made if you're grabbing more than a few lines. (Unless it's something real slick) I think this is such a hot topic due to Perl's cutNpaste nature.
O'Reilly probably doesn't care too much since they do make the code that it is in their books available for download. The Cookbook code is without any attribuation, Perl for System Admin has it.

On this group though, say where you've gotten the code so other monks can benefit from what else the source offers.