Natural languages are full of idioms and conventions that have little or no philosophical significance. For instance, it is true that 'perfectly powerful' is not idiomatic; we are more likely to say 'X has absolute power' or 'X is omnipotent'. ('Absolutely powerful' is not idiomatic, incidentally; the relevant idioms are those I've just indicated.) However, this is just a quirk of natural language. The following is an example in which 'perfectly powerful' is used with full philosophical justification and in which its lack of idiomatic currency is irrelevant:
The concept of God is the concept of a maximally powerful perfect being,
a being who, among other impressive attributes, has the maximal degree of
power it is possible (in a broadly logical or metaphysical sense) to exemplify.
God is thus ajperfect being who is perfectly powerful. To specify this further,
the traditional theist typically makes three claims. First, there are and can be
no independent, externally determined constraints on divine power. Secondly, the internally determined structure and scope of God's abilities to act
(those parameters on divine action set by his own nature and activity) are
not, and cannot be, such that he lacks any possible ability or power it is
intrinsically better to have than to lack. And finally, God is the sole source
and continuous support of all the power there is or could be. Thus, no individual can possibly exemplify or exercize any ability, capacity or power
whose existence does not ultimately derive from God.
(Thomas V. Morris, 'Perfection and Power', International Journal for Philosophy of Religion, Vol. 20, No. 2/3 (1986), pp. 165-