The proper forum would be rt.cpan.org most likely, in the queue for DBD::mysql more specifically.

That aside I'm pretty sure that most DBDs have limitations of some form or another (usually imposed by the underlying database libraries themselves) as to where exactly placeholders can be used. You didn't post what error message you're getting, so this is just a WAG.

Update: Changed phrasing about placeholders from "what can be used" to the above. Was thinking "what places you can use placeholders" and didn't clean up the draft.