It does rather depend on which perennial you want to grow. Most seed packets recommend April to June, which gives time for plants to grow to a reasonable size and be planted out in the autumn. Seeds sown now will often have to be cosseted through the winter for spring planting out. Some seed needs to sown fresh, and if it's ripe now then should be sown immediately. Others need a cold period to germinate, so sowing in late autumn might be the best time.

Thanks, Landgirl. Now all I have to do is find out which ones can be planted now. I suppose reading the seed packets in the garden centre would be a good starting point.

In the wild the seeds are just ripening on the plants and falling to the ground. This is the practice that I follow with all perennial seeds; I sow as soon as I obtain the seeds and, mostly, leave the pots outdoors over winter.
If these are packets of seeds that you have just bought, remember that this seasons crop are only just being collected so what you are buying are already a year old.