you can slip a bottle over it while it is small, it will grow inside. I have pear liquor with a whole pear inside the bottle & that's how they get it in there. The bottle is glass but plastic should work. You might want to use glass though because squirrels will chew through plastic(you'll have to break it to get the pear out). Maybe a little wire cage? I had a squirrel chew the handle and part of the lid off a rubbermaid tote last year trying to get to the chicken feed inside, they are relentless little critters.
I would find an old birdcage and hang it in the tree with one branch running through it maybe you'll get more than 1 pear in there.

If it's only a few squirrels, you could get a hav a hart trap (they sell squirrel sized ones) and get rid of them that way. Just make sure to get rid of them far enough away so they don't come back.