APL (Dyalog Unicode), 23 bytes^SBCS

Full program. Prompts for text and prefix from stdin. Prints list to stdout.

(5⌽'(\w+)\b',⎕)⎕S'\1'⊢⎕

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⎕ prompt (for text)

⊢ yield that (separates '\1' from ⎕)

(…)⎕S'\1' PCRE Search and return list of capture group 1 from the following regex:

⎕ prompt (for prefix)

'(\w+)\b', prepend this string (group of word characters followed by a word boundary)

5⌽ rotate the first 5 characters to the end; '\bPREFIX(\w+)'