Addendum: I always forget to mention this, but I really love the people at FanGraphs. That site is where I get most of my information for these types of analysis, and they are just wonderful.
After a successful season in 2007 where Perez went 15-10 with a 3.56 ERA, Perez fell to 10-7 with a 4.22 ERA despite pitching 5 more games and 17 more innings (only a little more than 3 IP/start?). I found that a little surprising when I first looked at the stats. His BABIP of .287 last season dropped to .280. His LOB% rose from 71.6 to 73.4. His GB/FB rate increased from .65 to .70. It almost seems like he should have been better. But then, I saw some other stuff. His K/9 dropped from 8.85 to 8.35, but the biggest thing was that his already high BB/9 of 4.02 went all the way up to 4.87. Perez's HR/FB also went up from 8.8% to 9.8%. In total though, he shouldn't have been much worse (at least not more than a half-run) than the previous season. However, Perez's FIP last season was actually 4.32 (4.68 this past season -- not a big jump). That was a BIG difference caused by his low BABIP and HR/FB. He's also been aided by pitching in pitcher-friendly and now extinct Shea Stadium.
What does the randomness tell us? One, he isn't a very good pitcher. The only consistency he has are his FIP's, which are always at least in the mid-4's except for his fantastic 2004 where it was 3.45 (but his BB/9 was also really below his career mark -- 3.72 vs. 4.76). Two, he's unpredictable. His high walk rate and flyball tendencies lead to inconsistency and luck playing a larger role (will the walks hurt -- LOB% -- and will the flyballs turn into outs or home runs -- HR/FB -- which may be a five foot difference). Three, he can't throw strikes. A new metric called F-Strike% (percentage of first pitches that go for strikes) is interesting. The norm is around 60-65%, but last season, Perez's was 52.9% (and usually around 55%, which is really low). If you can't get ahead, it makes it harder to get guys out. Four, he strikes out a lot of guys. His career K/9 is a really high 9.25, and teams just love power-guys who strike out a lot of people, especially lefties. Ollie is that guy, and that's why someone is likely to give him a chunk of dough this off-season. They just hope his BB/9 drops dramatically.
For those who want a Randy Johnson comparison, Johnson's first five BB/9 were 2.42 (4 games), 5.38, 4.92, 6.79, and 6.16 before dropping down to 3.49 and going down essentially ever since (How in the world did sub-4 ERA's those seasons?).
So, Perez could get his command under control (pun), but I'm not so sure he will. There is the possibility however, and I bet Boras plays that in his negotiations. Otherwise, I don't think I'd spend too much money on Perez this off-season, even if he's only 27.