Stress test fine but what does "perfusion is mildly nonuniform but appears preserved through the myocardium" mean? should i be worried?

No worries: Perfusion means blood flow so the report indicates that some areas of the heart were not as highly perfused as others (minimal difference since it was "mildly non-uniform"). But the perfusion is preserved through the myocardium which means that all areas of the heart are receiving adequate blood flow!

Nuclear imaging: I presume you are referring to a radionuclide stress test imaging report. The technique of the study can cause what your report says and usually does not represent a problem. There are anatomic abnormalities and variations which could also cause that appearance so although the likelihood is nothing to worry about, you can't get a guarantee from out here.