The phrase "a thing perfectly" is correct and usable in written English.
It can be used when discussing the quality or manner in which something is done, emphasizing that it is done without any flaws or mistakes.
Example: "She completed the project a thing perfectly, leaving no room for improvement."
Alternatives: "flawlessly" or "to perfection".
I don't do any one thing perfectly, but I do a lot of things just well enough.
Every now and then in New York City, you will come across a thing so perfectly useless it reminds you of an old idea you managed to forget: Superfluous things are often beautiful, and beautiful things are generally superfluous in the end.
"Let me make one thing perfectly clear," he added.
Rather than do a few things perfectly, accomplish many things successfully.
How great is that?! I don't know why this is so immensely satisfying for me, in a "things fitting perfectly into other things" way.
A lot of things perfectly falling in place is exactly how his life was saved that fateful day in 2014.
Under successive iterations of the Patriot Act, and the system of secret intelligence courts it has codified and extended, American intelligence agencies can do a lot of questionable things perfectly legally.
His line is impeccable, his turns a thing of beauty: perfectly centered and upright as he spins, the unsupported leg outlining perfect geometric shapes.
A pointy boundary, for instance, could be pointy in that it has no proper parts, but that does not rule out that it be located at an extended region short of assuming that the mereological structure of a thing always matches perfectly that of its location (what Varzi 2007:1018 calls "mirroring" and Shaffer 2009 138, Uzquiano 2011 "harmony").
When we encounter a mother doing too many things perfectly, smiling as if it were all so easy, so natural, we should feel a civic responsibility to slap her hard across the face and scream the word "Stop!" so many times that the woman begins to chant or whimper the word along with us.
