Kelvin Perez is a really small reliever (especially skinny, weighing maybe 150 pounds) in the Yankees AAA. His mechanics are pretty simple and seem repeatable and he throws 92-93 MPH (I saw a 97 MPH reading, but that may be a mistake. He mixed in a 94 MPH straight fastball though.) fastball with a little bit of tail with a 79-81 MPH slider with late hard break. He mixed in a 87 MPH change with a little bit of movement. It is almost a starter's repertoire, but he doesn't have the size (and has already had arm issues) to throw a whole lot of innings. He is somewhat of a minor league veteran with 7 seasons of experience and 26 years old. An arm injury really set him back but he has seemed to taken big strides. He showed some good control and he looked like a major league reliever to me if used in short spurts.
Chase Whitley was one of the more interesting arms in the International League this year. The Yankees prospect had a 3.70 FIP as a 23 year old. He was throwing a 92 MPH fastball as a reliever (been used as both in AAA) with a little bit of tail into righties. He mixes this with a 84 MPH change. I didn't see a 3rd pitch, which he will need as a starter.
Matthew Wisler was the best pitcher in the Midwest League by some metrics. The Padres prospect has a curveball with slow break. He can throw it for strikes, but it is not "sharp". He has a low 90s straight fastball along with a moving fastball. His command/control didn't look great when I saw him, but he walked just 6.1% of the batters he faced this year.