I don't generally like things that re-indent when I'm not doing something like saving the buffer (obviously) or hitting tab to request a reindent.
I like the automated indentation happen as soon as reasonable (e.g. after a return) on a small scope, so I can visually verify it's exactly what I want. I can get perltidy to do 98% of what I want, but there are rare times when I want a specific hand coded indentation, and I don't want it to change silently afterward. I'm probably just too picky or don't know enough about perltidy to rely on it 100% yet.