Why do you want a perennial garden? An odd question you may think, but the answers will lead to your garden goals. Goals guide us through a process. They keep us focused. They prevent wasting time, floundering through design decisions.
Upon completion of your garden you can compare the end product with your intended goals and see if they were met. Goals can change through out this process. Accept the change and discipline yourself to follow it. So again, why do you want a perennial garden? What do you want from a perennial garden? Common answers are to have pretty flowers or a pleasing view. These can be goals. If you want flowers only, then don't spend time researching foliage plants. If you have a beautiful view, try to enhance it and avoid tall plants that will interrupt or conceal it.
Achieving aesthetically pleasing gardens is always a goal, but is there a mood or a feeling you want to create? Will the garden have a utilitarian purpose.... herbs for cooking.... cut flowers.... dried flowers? Or will it offer sensory pleasure.... fragrance.... sound.... touch? This task of pinning down your gardening goals can take some time. Before you continue on, take a few minutes every day and think about what your goals are. Here are some questions to help you:
Your answers create your goals - you now have a list of reasons for wanting a perennial garden. Remember your goals to keep you on track. Goals can be changed. But remember, you will evaluate your gardens' performance based on them.