Karnatakabhashabhushana 2.17

Na, ṇa, la, ya, and ḷa, preceded by a short [vowel], [are doubled] twice before a vowel.

The sounds na, ṇa, la, ya, and ḷa preceded by a short syllable undergo gemination when followed by a vowel.

ponnuḷḷavane kulīnaṁ;

maṇṇellaṁ;

pullillaṁ;

keyyādudu;

muḷḷuḷḷuve kēdagegaḷ.

Why [is it said], 'preceded by a short [vowel]'?

nān endaṁ;

jāṇindaṁ;

mēlāḷgaḷ;

bāyāṟal;

kēḷītaṁ.