[enneff]

It seems like a nod to the “tr” command.

[jcul]

Yup, I guessed the same, and indeed it explicitly states so in the blog post linked by another commenter.

> trurl is a tool in a similar spirit of tr but for URLs. Here, tr stands for translate or transpose.

[obelos]

Though it hardly gets any use because s/// is so much more flexible, Perl also has a tr/// builtin that replicates the behavior of the command line tool.