Think a lot depends on what they do with Pierce.
If they decide to do a full-rebuild as soon as they can, Pierce may not want to sign an extension. If he walks (or gets traded), they can keep Perk around at his fair value (around $10M/year). Come 2012, they'd have Perk and Rondo on the books for ~$21M, plus whatever draft picks and other players are kicking around, cap holds and whatnot. There would still likely be enough room under the cap where you could pursue an impact free agent in 2012, even with Perk and Rondo on the books.
But... if Pierce signs an extension, figure something like another two years, $24M-ish, you'd be looking at Rondo, plus PP, plus Perk, plus the other cap holds and rookie deals and all that... suddenly, you aren't looking at having much cap space. Probably not enough if you were looking to make a run at Carmello or (*love to do this*) Durant.
So, if you decide that you MUST lock up Pierce, for all that he means to the franchise... do you really want to lock up Perk, as well?
Perk's a $10M player. Hell, he was (I thougt) more like a $7M or $8M player when he signed his old contract. Only question is whether the C's want to commit to his being one of the rebuilding blocks, or if his contract's going to get in the way of making 2012 a VERY interesting summer.