Be aware that there are some places where you can't use placeholders, even if your DBD supports them. The two which I hit most often are that you can't use a placeholder for a table name; and that you can't use placeholders for variable-length WHERE IN('foo', 'bar', 'baz') constructs. Another I have come across less often is the multiple-record INSERT variation.

This is because all of those alter the structure of the query, as opposed to just plugging different constant values into the same query.