I've seen several mint pear recipes. There is one using creme de menthe, but since you mentioned flavoring instead of liqueur, I'll go with those:

The old Kerr canning book had a "Minted Pears" recipe. Per 4 pounds of pears (prepped and quartered) make a syurp of 3 1/2 cups water, 1 3/4 cups sugar and up to 3 T. bottled or fresh lemon juice (optional), 1/4 tsp. green food coloring (40 drops). You can always add less than 1/4 tsp. if the syrup has already become the desired shade of green.

Hot pack the pears by adding to the boiling syrup and bringing back to the boil. Remove from the heat.

At this point you have two options: One Kerr edition says 1/2 tsp. mint extract per batch of syrup; another Kerr edition says 1/4 tsp. per pint and 1/2 tsp. per quart. I guess it depends on how minty you want them.

Fill jars with pears and syrup, processing as per usual for hot-pack pears.

If you prefer to use oil of peppermint, the only directions are to add the oil 1 drop at a time to the syrup until desired strength is reached. I've used essential oils, though not for pears. All I can say is "less is more." A little essential oil will go a long, long way.

Carol