I had this (weird) bug on my machine:
MPEG-4 encoded video sources that are converted to other formats (through Quicktime Player Pro, or iDVD) had only every other frame updated, resulting in strobe-like video output despite the frame rate being "normal" speed. The playback was fine, only the results of a "write to new output" was affected. This disappeared once i disabled Perian by moving the component out of the Quicktime plugins folder.

This may well be caused by other codecs interfering with Perian, not a bug in Perian per se, but i've not (yet) investigated further.