Include the dependency.

As discussed in this recent posting on perl.module-authors, this is not something I always recommend or practice. I've done it different ways for different reasons.

But given: (a) that your code's intended audience is likely to be very Perl- and CPAN-savvy and (b) that you are already requiring a lot of non-core CPAN modules in the production code -- modules that are much less familiar (to me, at least) than HTML::Diff, I see no reason in this case to be shy about introducing a test-time dependency.