Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
perfectly [ˈpəːfɪktlɪ] adv (emphatic) вполне́, соверше́нно (faultlessly) безупре́чно; (completely) вполне́, прекра́сно
I'm perfectly happy with the situation я вполне́ дово́лен положе́нием дел
you know perfectly well Вы прекра́сно зна́ете
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: