Seen today on I St. NW, next to the Department of Veterans Affairs:
For those not immersed in the culture, a brief explanation. In the LISP programming language, "car" takes the first item in a list, "cdr" takes what is left over. The programming language Perl is ubiquitous on machines running UNIX and Linux; the pathname of the executable is generally /usr/bin/perl. This sticker seems to have Linux in mind, with a penguin dressed up in knitted hat and scarf.