Well, if the format of the data input changes, then there is a chance that you have to change your program.

Maybe the XML::LibXML program will have to be changed, or maybe the rergex program will have to be changed (or neither or both). Nobody can know for sure which one, it depends on the nature of the change.

I know some people will probably shout at me for that, but in such a simple case, I would probably go for a regex. You don't need (and don't want) a cruise missile with an H-bomb in it to kill a mosquito on your arm.