It's not spam. It's a relevant mention of how to obtain the book that is the subject of this thread at a 50% discount from the publisher (or any other Perl book from that manufacturer). Mention that these sorts of discounts come up again from time to time, and you have a post that is still of value after the sale expires in a few days. Seriously, people need to lighten up if they consider you helping people reading this thread to obtain the book at half the price to be spamy behavior.
O'Reilly also offers discounts to members of Perl Monger groups by virtue of PM's affiliation with OSCON, if I'm not mistaken.
The Monastery is about enhancing peoples' education with respect to Perl. Let's not prohibit the mention of a book sale in a thread that talks about a book aimed at achieving a compatible goal.