IIRC, the DBI spec doesn't support using placeholders for the column names, only values. Programming the Perl DBI, p 221 says the following:

With most drivers, placeholders can't be used for any element of a statement that would prevent the database server from validating the statement and creating a query execution plan for it. For example:

"SELECT name, age FROM ?"       #wrong (will probably fail)
"SELECT name, ?   FROM people"  #wrong (but may not 'fail')
[download]

While this may not fail with some databases like MySQL, it *may* fail on databases like Oracle that natively support placeholders. (can anyone confirm this?)