Actual data on the ineffectiveness of common “perks”?

I'm trying to make a presentation about employee retention and I was looking for some actual data from a good source on how “perks” like free lunches, t-shirts, and candy aren't effective in employee retention. I've done some googling but I'm having difficulty finding data on this specifically and seem to just get articles with subjective reports. Any direction would be helpful! ​ Thanks!