Karnatakabhashabhushana 2.14

[Sūtra 14:] After a vowel, y, r, and l, v is diversely [substituted] for the pa-class.

After a vowel, after y, after r, and after l, the sound v is diversely the substitute for the pa-class [following] a vowel.

[Examples:] posavesar,

eḷavidir,

maravaṇe,

keyvaṟi,

meyvesaṁ,

tāyvane,

balvaḍe,

kalvāḻe,

pulvane,

nīrvāpu,

nīrveḷasu,

nīrvāsanaṁ.

Because of the word “diversely”, in some cases it does not occur.

[Examples:] niḍuden,

uḍuvogaṁ,

kemmāḍidaṁ,

keymaṟiyaṁ,

perbāgil,

kārmmugil,

balmuguḷ,

pulmaruḷ.