In my opinion, if you are >50% thinking that your cham did eat the piece, I would start calling your vet *NOW*. Depending on how big your cham is in relation to the piece of plastic, a vet hospital can probably get an idea of something foreign (ie, the plastic piece) in your cham's gastrointestinal tract by taking radiographs. I recently had x-rays done on my little Carpet (we're talking an 18g cham) and they were diagnositic for what we were looking for (unrelated problem to this, but my point being, your cham and the plastic piece might be big enough to differentiate on x-rays). If it appears the piece was indeed ingested, it might be possible for your vet to remove it surgically, before the piece lodges in the lower GI tract where the diameter of the GI lumen is smaller and more "tearable", or before the cham tries to regurgitate it up and cause damage to his stomach, esophagus, mouth, etc. It would also be advisable to try surgery before the chameleon becomes debilitated, whereby the risks of anesthesia and recovery are much worse. A healthy chameleon should be able to recover well from simple surgery, contrary to anecdotal beliefs that cham's generally don't do well under the knife for whatever reason. The common problem with chams undergoing surgery is that most of the time the cham is so sick from whatever ailment that a vet is consulted only after the cham is seriously ill, and the hapless vet gives the option for surgery as a "last resort". My veterinarian and I (I'm a lic. vet tech) recently performed major abdominal surgery on an ailing female Veiled taken in as a rescue. She was hopelessly eggbound without surgery, and once inside we found horribly cystic ovaries in addition to 15+ eggs in various stages of development. We had to spay her in addition to "egg-ectomy" and after at least an hour under anesthesia (that was my job) she woke up just fine. She was not in the best shape entering surgery, either, I have to say, suffering from long-term MBD that egg-development definitely wasn't helping. So my point is, if you consult with a DVM and he/she says surgery might be the only option, don't automatically think your cham will die; there are skilled exotics surgeons out there, mostly the problem is just finding one confident enough to operate on your little reptile! And that's just the worse case scenario, your vet may be able to remove the piece with endoscpoy (like giant tweezers enclosed in a tube that can be gently put down the esophagus into the stomach and grab the foreign body), or he/she might say there's nothing to worry about. But in my opinion, I wouldn't wait to consult a cham-friendly vet, and take with you a piece of plastic similar to the size and material that you think the cham ingested.
~Kerry