The critical techology in WATIR isn't the language in which it is written. The critical technology is the sophistication with which the framework is capable of manipulating the Document Object Model (DOM) in Internet Explorer.
The WATIR people are doing an outstanding and elegant job of this in Ruby. There's no reason it couldn't be done in Perl, but no one has yet, at least at the level of elegance and sophistication demonstrated by WATIR.