Tom Christiansen (also nicknamed "tchrist" or occasionally "thoth") is a well-known Unix developer and user especially known for his many contributions to the Perl programming language. He was the author of much of the core Perl documentation, including the manual pages perlfaq and perltoot. In 1999, he was one of the original recipients of the White Camel Awards from Perl Mongers for his contribution to Perl's documentation.
Christiansen was one of the early contributors to Perl after its public release in 1987. He co-authored the The Perl Cookbook, the second and third editions of Programming Perl, and the second edition of Learning Perl (and its spin-off Learning Perl on Win32 Systems).
The common phrase "Only perl can parse Perl" is attributed to Tom Christiansen, although it probably originated from "Only tex can understand TeX". Randal Schwartz also credits him with accidentally naming the Schwartzian Transform for optimizing some types of sorts. This happened after Schwartz used it in a Usenet message, and Christiansen replied to the message giving some corrections and in one place said "the Schwartzian transform" to refer to the transform that Schwartz used.