Fringe benefit (5)
'fringe benefit' is the definition.
(synonyms)
This is all the clue.
(Other definitions for perk that I've seen before include "Incidental benefit from one's employment" , "Company car, for example" , "Fringe benefit" , "extra privilege" , "Cheer" .)
(Other definitions for frill that I've seen before include "Shared feature of Mr Punch, Richard III" , "Ruff, adornment" , "Pleated strip of cloth trimming or an unnecessary extra" , "Extra embellishment" , "Ruche; ruff" .)