Age. O'Reilly most likely has decided it is too old to matter any more. It used to be available on CPAN also, but if it is even there any more, you'll have to dig really deep. (Unfortunately it's not the time now to check it out, with the current status search.cpan.org is in.)

Anyway, with a little of Googling around, you can find plenty of mirrors, like here.

I do share your sentiments, though, IMO it is a classic. I'm actually planning on building a site with a collection of such perl related classic articles, before they drop off the next completely. Like this one virtually has.