Intertran lists the following translations:
berffeithio, mherffeithio, perffeithia, perffeithio, perffeithiwch, pherffeithia, pherffeithio, pherffeithiwch, 'n anhyfreg, 'n berffaith, 'n ddiwall
Assuming that you require 'perfect' as an adjective, rather than as a verb, 'berffeithio' seems to be the best option. For example, Intertran changes 'a perfect day' to 'a berffeithio ddiwrnod'.
http://www.translation-guide.com/free_online_t ranslators.php?from=English&to=Welsh
Chris