Without doubt, Geraniums (Cranesbills) They flower in colours pink, blue, violet, mauve or white depending on variety. They thrive in most soil types in semi-shade and once established need little watering and feeding. All you have to do is cut the spent flower stalks back to encourage more flowering.

They vary from ground cover to plants 3' high. The foliage of many of the deciduous varieties turns red or orange in autumn which is an added bonus. I would recommend Gearanium 'Brookside' (blue flowers, white eye) which flowers for months from June. Alternatively any of the Geranium Himalayense varieties are good. A shorter flowering season and height than Brookside, with mauve flowers. 'Plenum' is the most popular of the Himalayense which has double flowers.

Do you really mean perennials, as in herbaceous perennials, which grow, flower and then die back down beneath the ground in winter, leaving the area completely devoid of plants?

Perhaps you'd prefer some small/medium shrubs mixed in - some are evergreen and even those that aren't have a woody skeleton above ground. Shrubs are the lowest maintenance plants - perennials need to be lifted and divided every 3 or 4 years.

I won't make suggestions till I'm clear precisely what you would like - but in terms of planting using membrane, usually, you would dig over the area, incorporating composted manure or composted materials such as garden compost, leave it to settle for a week, level off and lay a membrane - you then cut through that where you want to plant, making a cross shape, peel back the quarters and plant through the hole, reshape the liner round the plant. Mulch on top goes on last, after you've planted. That's another thing - perennials don't cope too well with membranes - they want to spread through the ground and put out more topgrowth, and they're restricted by the membrane surrounding them so you never get good ground cover over time. Plus many of them have a habit of dying out in the central portion, growing instead further out round the outside, again, something that can't happen with a membrane.

Welcome to GoY Deed, quick question from me when you said the area 'used to be a pond' did you either make a lot of puncture holes in the liner, remove, or is it still in place?

Hopefully the pond was drained prior to filling?
Geraniums are my immediate thought as Bendipa said but Bamboo makes a good point.

Heuchera's except for the dreaded vine weevils

Geraniums would be great(Not to be confused with the bright red and pink ones used for summer bedding - those are actually called pelargoniums but are sold as Geraniums by some suppliers). Geraniums come in many shades of blue or pink. Most die back completely in winter so its a good idea to put some bulbs between them. I'd avoid membrane - weeds will seed into the mulch you cover it with so you are no better off after a year or two.. The geraniums will spread in a few years and cover the area completely. Look at Long Acre Plants website for pics. You could put in some crocosmia for contrast - they would only need thinning every few years if you felt the clump was getting too big. A fern or two would look nice and would look after themselves: Dryopteris or polysticum setiferum should be OK - the latter doesn't mind dry shade which is unusual for ferns.