I'm sure I've seen this mentioned before, not too long ago, on the DBI-users mailing list. I think this must be a bug in the DBD driver. I've tried searching though the archives, but no luck: I don't really have many valuable keywords to search for. I don't know if this got fixed in the latest driver...

Your best bet would be to upgrade to a recent version of DBD::mysql, and if that doesn't help, maybe ask about the status yourself on the mailing list. There are many smart peoplen, even quite a few of the developers who (co-)wrote DBI or one of the DBD::* drivers, are on that list. If anybody can give you an answer, they are it.