The definitions in the Collins COBUILD indicate that these two phrases,
fringe benefits and perks, can be
used interchangeably (an example in both is "a car.")
The American Heritage Dictionary (p. 728) defines fringe
benefit
as "an employment benefit given in addition to one's wages or
salary."
It defines perk as "a perquisite" and lists
its usage as "informal." (Quotation marks here and later for
"informal" are mine.)
On page 1350, the dictionary defines "perquisite" as "a
payment of profit received in addition to a regular wage or salary,
especially a benefit 'expected' as one's due." (Quotation marks
here and later for "expected" are mine.)
I think the key words here in the definitions of perk
and "perquisite" are "informal" and "expected."
There is no doubt that perk is informal; it would never
appear in a serious written document, while fringe benefit
might.
Also, a fringe benefit would be defined, and it would
be established throughout a company. It could include discounts on purchases
on the company's products, for example, or the right to park (even if
you have to purchase a parking sticker) in the company's parking lot.
A perk, on the other hand, is less well defined; it
is something expected by people with certain privilege in the company.
Perks might include a reserved parking space,
or a corner office with a view, or membership in a private club. Perks
would probably not be defined in writing.