The problem is trademark, not copyright. O'Reilly owns the trademark on it in the context of Perl: "The camel with the topic of Perl is a trademark of O’Reilly Media, Inc." -- https://www.perl.org/camel.html

The camel picture used on the original book cover is in Animals 1,419 Copyright Free Illustrations of Mammals, Birds, Fish, Insects, etc. by Jim Harter: https://www.doverbooks.co.uk/animals-1419-copyright-free-ill... (image 241).