Perron, 25, had 10 goals and 15 assists for the Blues in 48 games this past season.
A first-round draft pick in 2007, Perron scored a career-high 21 goals despite playing in just 57 games in 2011-12. He has 84 goals and 114 assists over parts of six seasons, all with the Blues.
"I think that David, he's a dynamic player, and he has an unbelievable skill-set that sometimes takes a little bit of time to get used to playing with ... for his teammates to get to understand his nuances," Blues general manager Doug Armstrong said on a conference call.
The 22-year-old Paajarvi, the No. 10 overall pick in 2009, had nine goals and seven assists in 42 games with the Oilers this past season.
"We view him coming in and competing in our group of nine," Armstrong said. " Our team is built on balance and we think he has an opportunity to come in here and provide an element that we don't have right now."
Information from The Associated Press was used in this report.