You've asked what constitutes a "big" project in Perl and I can only say that this is terribly subjective. One project I worked on was a quarter million lines of code (pretty good code, too) and that was definitely a big project. Were it written in Java or C, it could easily have topped a million lines. There are those who will swagger in and say "yeah, I maintained a five million line project" and there are those who maintain 50K lines and think those are huge, but there's really no arbitrary cut-off. Thus, to say that Perl's bad for "large" projects does beg the question.
That aside, we had no problem maintaining our quarter million lines of code because we pair-programmed, had decent test suites, respected encapsulation, didn't have (many) unrealistic deadlines and so on. In short, with competent programmers and a decent work environment and code base, Perl scales well and we could get things done fast, fast, fast.
That being said, finding competent Perl programmers turns out to be more difficult than one might think. This is a constant complaint I hear from employers.