DBI always supports placeholders (even if the underlieing database doesn't)

While this is mostly the case there are exceptions. One that is fairly significant given the number of people using MS-SQL is DBD::Sybase with FreeTDS, which does NOT support placeholders at the moment, because of a lack of support for placeholders in FreeTDS's version of the Client Library API (which is what DBD::Sybase uses.)

Other systems or combination of front-ends and database engines may have similar problems.

Michael