The moment that you become a PR, any Canadian visa or work permit that you possess is automatically void, regardless of them physically taking it from you. They should void any visa in your passport and keep any work or study permits if you have them with you.
when I went to activate my new permit, they were going to take my old one, but I asked to keep it, which was no problem, they drew a line thru it nd wrote void and gave it back to me.