This is (was?) O'Reilly's stance on the matter:
https://web.archive.org/web/20180425080044/http://archive.or...
The Perl Camel Usage and Trademark Information
As most of you probably know, O'Reilly started putting animal images on the covers of our books about thirteen years ago. To millions of readers, the animals mean O'Reilly. They've become our signature "trade dress." We've also trademarked the association between particular animals and the subject of their books. After all, the only reason that people think of camels in association with Perl is because we used a camel on the cover of Programming Perl.
We recognize that things do get more complicated, though, when an image like the camel is so widely known that it comes to symbolize not just our products but also the entire Perl language. This is a good thing, and we want it to continue. But trademark law is sticky on this point. If a trademark isn't "protected" (by letters asking people not to use it, or by licenses that allow them to use it only in specific ways), it gets into the public domain and loses its protected status. If this happened, anyone could use the camel without restriction, including in ways that were detrimental to the language. For example, you might imagine a company creating a Perl-compatible language, branding it with a camel, and pushing it as the "official Perl" in an attempt to drive Larry Wall's Perl out of existence.
Another important issue is that a brand is strong in proportion to two things: its ubiquity and its distinctiveness. It's important that, just as we want one version of Perl (so we don't have the fragmentation that was the downfall of UNIX), we have one symbol for Perl. To protect the integrity and impact of that symbol, we need to maintain some artistic control over what kinds of camel images are used. We believe that "one camel" will strengthen the overall Perl brand.
In short, we're walking a fine line, trying to make the camel as available as possible as a symbol for Perl while protecting it as a trademark. So, here's our policy on using the camel image:
Non-commercial use
We will license the camel image widely for open source products and non-commercial sites related to Perl, requiring only an acknowledgement of its trademark status and a link to www.perl.com. To request the camel artwork, please send email to permissions@oreilly.com, indicating where, how, and for what purpose you plan to use the image. Please note that we generally do not allow alterations of the Perl camel artwork.
Some non-commercial sites currently using the Perl camel:
(snipped)
We also offer the Programming Republic of Perl logo for some non-commercial sites. Feel free to download these logos for use on your pages. Please make the logo a link to www.perl.com.
Some sites using the Programming Republic of Perl logo:
(snipped)
We may also license the Perl camel image for some commercial products and sites related to Perl. To inquire about the use of a camel image on any commercial product or site, please send email to permissions@oreilly.com with a description of the product or web site, indicating where and how you'd like to use the camel.
We've also created "Powered by Perl" buttons that any site using Perl may use on web pages. Feel free to download and use these buttons. Please make the buttons link to www.perl.com.
And the Camel FAQ:
https://web.archive.org/web/20180123132933/http://archive.or...
Q: So are you saying that O'Reilly has trademarked an entire animal?
A: No. When a company receives a trademark, it receives protection for a symbol in a particular category of products or services. For example, Owens Corning has trademarked the color pink. The whole color? No, only for insulation. O'Reilly has protected the camel image for books and online publications related to the Perl language, and related product and services. The only reason an association exists between camels and the Perl programming language is because we've used a camel image on our Perl-related products.
Q: Do you just own the particular Camel on the cover of Programming Perl, or all camels?
A: We own the particular camel image shown above, which has lead to an association between camels and the Perl language. If someone were to use a different camel on their Perl book, there could be confusion over which one "The Camel Book" referred to, and we might need to step in and stop use of that camel image. That's how trademarks work, helping to protect confusion in the marketplace.
Q: I want to design a T-shirt with the Perl camel on it. Do I need to get your permission?
A: Yes. But we're willing to make allowances for those of you who have creative ideas and want to do something fun for your friends. So, if the lifetime print run of the T-shirt design is less than 100, you may consider permission automatically granted. For larger print runs, please ask first. We promise to answer quickly!
Q: Why isn't your trademark just restricted to books?
A: We also do conferences, software, research, and online publishing in Perl, and we use the camel image for those things as well. We may want to camel-brand other Perl-related products in the future.
Q: I want to use $camel as a variable name in a Perl program. Do I need to acknowledge the trademark?
A: No.
Q: I want to use a cartoon camel as the logo for my software product. Is that okay?
A: It depends on what your product is, how it was developed, and how you intend to distribute it. Please send email to permissions@oreilly.com, with information about what you'd like to do, and we'll get back to you.
Q: I want to place a picture of a camel on my Perl web page. Am I allowed to do that? Do I have to use your camel?
A:Yes, as long as your page is non-commercial, and the context in which the camel is placed portrays Perl in a positive light. You will need to include the following language in small text somewhere on the page where the camel appears:
"The Perl camel image is a trademark of O'Reilly Media, Inc. Used with permission."
Please make the "O'Reilly Media, Inc." part of the statement a link to our home page (http://www.oreilly.com).
We'd encourage you to use the Perl camel we use, as it has wide recognition as "the Perl camel." But if you have another camel you'd like to use on a non-commercial site we generally would not object, so as long as the image is in no way derogatory.
Please note: If you use the "Powered by Perl" or the "Programming Republic of Perl" buttons, please make those active links to http://www.perl.com, not the O'Reilly home page.
Q: What is the Programming Republic of Perl logo?
A: The Programming Republic of Perl logo was developed some years ago for non-commercial use on web sites, and serves as a pointer to www.perl.com. Feel free to use it on any non-commercial pages. You can find it on the main Perl Camel Usage and Trademark Information page.
Q: Where can I find out more about camels?
http://www.sandiegozoo.org/animalbytes/t-camel.html
If you have questions or comments about the Perl camel or any other O'Reilly trademarks, or if you want to use one of our trademarks in some way that we haven't explicitly described on this page, please send a detailed request to permissions@oreilly.com. For more information, see the Perl Camel FAQ.