The problem is not the copyright. They have a copyright on that purticular drawing of a camel. They also have a trademark on the use of a camel for the perl language.
Trademarks are strange things in some ways. Trademarks cover anything "confusingly similar". That would certianly cover the use of a different camel for the perl language. Similarly, I couldn't create a computer compony named Internal Busniess Machines, and call myself IBM, because somebody might confuse me with the real IBM.
Even stranger, is that trademarks must be defended, or they will be lost. That means that even if O'Reilly wanted, they couldn't let just anybody use the camel, without their explicit permission. If they do that, they can no longer keep the people they don't want to use the camel from using it -- it falls into the public domain.
http://perl.oreilly.com/usage/ gives more info on O'Reilly and the camel. For more information on trademark law, google is your friend.