Perennials in the YUKON!
I'm afraid that you're out of luck when it comes to magnolias in the Yukon, unless you provide special winter protection for them. But birches are a good bet. You can find a list of plants hardy in your Zone by using the advanced search option in our plant finder encyclopedia at http://www.bhg.com/gardening/plant-dictionary/advanced-search/. (You're in Canadian Zone 1a, but I believe that equates to USDA Zone 2. We use the USDA Zones.)
The other good news is that if you have reliable snow cover, you may be able to grow perennials that are much less hardy than would survive in the open, because snow is such an excellent insulator. A colleague in Quebec City (USDA Zone 3, grows plants usually hardy only in Zones 5, 6, or 7, but gets away with it because his garden is covered with several feet of snow every winter.