Thanks for writing. Falling leaves typically don't harm most perennials if you rake them up in early winter. You can rake the leaves into your lawn, run them over with a lawnmower a couple of times, and then sprinkle them back over your garden for a wonderful winter mulch that will continue to improve your soil over time.
There aren't any special tricks to growing perennials in these kinds of conditions except for choosing varieties that like shade and tolerate soil that's a little on the dry side (unless you water them extra) as tree roots absorb lots of moisture.
---Justin, Senior Garden Editor, BHG.com