What are "r" controlled vowels? And do the following words acceptable? fern, dinner, herd, prefect.

"R controlled vowels" are those that are followed by an "r" and do not make their regular short or long sound.

ar (park), er (certain), ir (circle), or (more), and ur (turn) and sometimes yr (like zephyr) are all of them.

All of your words have "r" controlled vowels except prefect.