[warwick]

You tell your boss something along the lines of "I've never worked with Perl before, and I don't think I'm going to be able to complete the project with the time we've budgeted, since it can take some time getting up to speed with a new language." Then you present options iike using a language you're comfortable in, or lengthening the time that's available for the project.

Often part of a technical job, particularly a technical job with any interesting work or responsibility, is educating your boss about the true costs associated with a decision so that they can make good decisions. If they want you to do it in Perl, that's fine, they're paying for the time and you've informed them why it will cost more than they thought. It's a good idea to follow up with an email restating this so that there's a written record you can point to later when the project falls through.

Honestly, I had expected that you'd refused a project that was unethical or illegal, not that you'd just decided you weren't interested in the stack they wanted you to use. If you told me that story during an interview, I certainly wouldn't hire you. It shows a level of inflexibility as a developer that makes me seriously question if the reason you refused the work is because you're not capable of picking up another language.