Question

most of the rivers of North India are perennial rivers while the rivers of South India are seasonal

Answer 1

This is because the rivers in north arise from Himalayan glaciers which are a permanent source of water.
Whereas there is no such source in the south .

Answer 2

Perennial rivers (aka permanent rivers) flow throughout the year except during extreme drought. Non-perennial rivers are those which have no flow for at least a part of the year.

Perennial rivers usually originate from mountainous snowy regions or glaciers. In India almost all perennial rivers originate from the Himalayan range.

There are 10 perennial rivers in India most of them in the north. They are Ganga, Yamuna, Indus, Brahmaputra, Narmada, Mahanadi, Tapti, Garghara (Saraswati), Sutlej and the sole perennial river in the south -- Thamirabarani.

Thamirabarani, the sole permanent river of the South, merits a few words.

Thamirabarani starts flowing from the peak Agastyarkoodam in the Western Ghats hills in Papanasam of Tirunelveli district, Tamil Nadu. Its old Tamil name is Porunai. Its 125km run ends in the Gulf of Mannar