But, that is not what you asked...... the error is pretty clear in most cases, except you appear to have run the execute; however, you have run it in a very unorthodox way (at least according to both the docs and the way I normally see it). You wrote:

my $rc = $sth->execute('OH');

Try:

$sth->execute('OH');

Even if that does not fix it you don't need the equation as far as I can see. I am not sure WHY that would cause it to fail, but it is out of the ordinary so I would point to that as my first guess, something to do with scoping I would think.