Certified GKH Gardening Expert
As long as you don't spray the pear tree, the chemicals, such as glyphosate, should not be absorbed into the tree. You can hold a barrier such as cardboard against the tree while you spray.
If you are going to dig it out, be very careful you don't dig into the pear tree roots. The pokeberry taproot is long and fleshy and it's hard to get all of it.