Though I would understand both in written and oral communication, I find the first to be pretty odd. I wouldn't ever use it, and strongly prefer the second, "according to".
Why is this so odd? I looked a bit in the OED at the use of "as per" (odd in itself for combining two prepositions). It was first used in 1869 as a "slang" form "as per usual" by itself. I did however come across an entry that has a similar meaning to what you're looking for here: "by".
By. 3. a. According to; as stated,
indicated, or directed by, as per
advice, per instructions, per invoice,
per ledger, etc. Usually preceded by
as.
This was used as early as 1446 and as late as 1989. The difference here, I think, has been touched on: this meaning implies some obligation or requirement. No other entries seem to come close, and this matches my own mental lexicon for "per" and "as per". It just doesn't fit in this situation.
From the Oxford Corpus of English:
PER ANNUM
For 30 years, it had gone remorselessly ahead, at about 80% per annum.
An Elan bond, where the bondholder can exercise the right to be repaid in 18 months, currently yields 19 per cent per annum.
With this fresh impetus, the total edible oil processing capacity, including vanaspati, is expected to cross 20 million tonne per annum.
PER YEAR
We can currently get about 5% per year from investing in long-dated gilts, so we might aim to get 6% per year from the property.
Penetration of digital has hit 20 % in a year, with incremental revenue per subscriber at $22 per year and growing, he said.
This still leaves teachers $3,000 per year short of accountants, $17,000 short of computer systems analysts, and $25,000 short of engineers.
These example sentences are representative of the most common uses of these two phrases and, as one can see, there is no real difference between per annum and per year in usage.
As kiamlaluno says, per annum is traditionally used more in financial contexts than per year, but these sentences show that per year is also perfectly acceptable.
Best Answer
The latter describes your salary: if you sit at your desk and don't get fired, you'll get $500. The former is more a description of the situation: somehow, by hook or by crook, I manage to scrape together $500.
The use of "per" imply a causal connection between the passage of time and the arrival of your pay. "Each" encompasses the possibility there's no connection, and it's just happenstance.