Shri Chandrasekharan,
Kala sarpa dosham arises if, as Shri Gopalan says, planets including or excluding lagna are situated in the horoscope between Rahu and Ketu. For a people who believed (and still firmly believe) that eclipses are caused by Rahu or Ketu swallowing the sun/moon, this placement of all 7 planets in between Rahu-Ketu would have immediately struck as the swallowing of all the planets by the R-K duo. fyi, there is a school of opinion which holds that only if Rahu is in the end it is complete Kala sarpa dosham but if it is Ketu which is in the front (Ketu to Rahu anti-clockwise) it is not very bad.
Astronomically a good percentage of people will have this kala sarpa dosha because one Jupiter and Saturn come in the same side of the Rahu-Ketu axis, then the rest will also be in the same segment - sun for six months, moon for 14 days each month, mars for abot 9 months and venus and mercury close to sun being in the same segment for a few months. Imagine the crores of human births during this period; if all of them start making stone naagar idols, what will be the result? Even hills and mountains may disappear in 100 years!
So, unless you have any serious problem in life, please don't go by what astrologer or pundit says. If at all you want to do parihaaram in order to get piece of mind, go to any navagraha temple and worship. After all Jawaharlal Nehru had this same dosha in his horoscope.