Perlon is a type of nylon invented in the 1930s. Perlon straps are single pass straps made from interwoven nylon cords. The look soft, but are slightly more robust than you might expect.
The real advantage of them is that you can put the buckle at any point along the strap as there aren't any predefined holes. This is massively advantageous to getting a comfortable fit.
They tend to come with a rather tacky stamped metal buckle. But it works well - the end of the strap is attached to it in such a way as to be adjustable. So the overall lenth of the strap will always exactly suit your wrist size.
Perlon straps also don't seem to have the issue of height that other single pass straps have - the material is so thin and pliable that the watch basically sits at the same height as it would with a standard strap.
All in all, a must have, especially for watches with fixed bars.