I have never had them blow up.
I think it is because the rum is already fermented, the fruit doesn’t ferment, it is like it gets pickled and just oozes flavor and aroma. Maybe that is why you need to use firm skinned, whole fruit.
I am no expert though, I only do what I saw my grandpa do without any other foundation than that, and I have to admit to peeking periodically (well, tasting 
) so maybe that is a contributor to success? Never thought of it that way. Don’t know if grandpa snuck into the cellar to nip, I wouldn’t put it past him. 
I, too, have pear trees and while I don’t do the trick with the bottles (no time) I make this stuff almost every year. Usually I just go for a pure, clear pear or grape flavored liquor with the most heavenly aroma, but sometimes I do add sugar to make a syrup that is super over ice cream (as is) or pork or in curry sauce (cooked). When I add sugar I taste more frequently to see if it’s ready and am not sure if it would blow up if I didn’t.
I have tried a lot of commercial fruit flavored liquor (the clear stuff, not liqueur, which is sweet and in my opinion the sugar kills too much flavor), especially Austria makes a lot of nice varieties, but none of it ever even compared to the flavor you get like this.
The clear stuff is at it’s best if you drink it like good Brandy, in a large, curved, warmed glass, by itself or after dinner.
I never make enough, although as a moderate drinker I just have a couple a week tops. I always end up giving some away.
Maybe I should get into mass production and sell it on the internet. 