The spring perch is simply the seat that the spring sits on. When you install factory (regular) style spings, they sit on the metal seat and then they are topped with the upper spring bushing and mount. However when you install CO they have they're own lower mount which is movable up/down, hence the adjustablilty. This lower mount is installed on a threaded sleeve. In order to install the sleeve you have to remove the factory lower spring seat (spring perch). I know you probably know most of this, but I put it here in case someone else doesn't. Hope this helps a little.
Joe
97 GS w/ Hahn stage 2
Keep cruisin' & keep the shiny side up.
http://joeseclipse.homestead.com