First, a couple of remarks for those not familiar with the Italian educational system, to provide more context (the OP studies in Italy according to his user profile):
- In Italy, there are really no final exams: exams are distributed along the year in 3-4 sessions, and students can decide to take an exam much later than the end of the course. Thus, professors always receive students' comments before the exams (there's no "after").
- The kind of survey described by the OP was common in my university up to a few years ago (now, it's managed through a learning management system), and -- yes -- professors received handwritten forms (e.g. I read the You must die remark described here on a handwritten form). The survey should be typically handed back in class, so students can't hand it back after the exam as suggested in one of the comments. In the edit below I explain in more detail the mechanism.
Should a student mitigate his/her judgment in the survey for fear of a harsher revision of his exam?
From my experience, I've never seen anyone wasting time in trying to pair the calligraphy of the remarks with that of the exam papers (I surely didn't), and I'd consider retribution extremely rare (though jerks exist). Therefore, I wouldn't bother mitigating your judgment, but be polite and keep it at a professional level.
Note: How evaluations used to work in my university before the introduction of a learning management system
(and how they still probably work in the OP's case)
Teaching evaluations are managed by a university office (not a departmental level). In the paper era, toward the end of the courses, a professor would receive two sets of forms: one contained a questionnaire prepared by the competent office; the other could be filled with free comments.
During one of the lectures, the professor would hand the forms to the students and would give a (20-30) min break to fill them in. At the end, a volunteer among the students would collect the two sets and put them in two different envelopes. The envelope with the questionnaires would be closed, signed and delivered to the office by the volunteer; the other envelope with the free, handwritten comments would be handed to the professor in class.
So, the professor could read the comments before the end of the course and before any exam.
Nowadays, we can still read the comments before the exams (as I said in the first remark, there's no way out for this), but they are no longer handwritten.