The Fatbrain affiliates program has been discontinued. As of 3/31, it has been replaced with the Barnes & Noble affiliate program. Barnes & Noble offers a 5% commission (or 7% if you sell $1,000,000 worth of their products); amazon offers up to 15% with a directly-linked book (as the dbi.perl.org site uses).
Or... perhaps the person who put up the link at dbi.perl.org was considerate and conscientious enough to check prices... Programming the Perl DBI is $27.96 on Fatbrain/Barnes & Noble, and $24.47 on amazon.
I hope it wasn't construed that I condone the patents (software patents stink), but if a site maintainer chooses to offer only amazon links for whatever reason, a visitor can easily note the isbn number and go to another bookstore if it's their personal choice to boycott amazon.