ORA-12704: character set mismatch despite US7ASCII

[Dan Osborne]

I'm getting the above error on a new machine set up with perl 5.8.0
DBD-Oracle 1.16 and DBI 1.46.

This triggers the error ...

        $cursor=$dbh->prepare(<<__SQL__);
                select  data_type,
                        data_length,
                        data_precision,
                        data_scale
                from    user_tab_columns
                where
                        column_name = ?
                and
                        table_name = ?
__SQL__

my $res=$cursor->execute(@fld);

Oddly, if I substitute literals for @fld it works OK ...

my $res=$cursor->execute('gen_user','gen_users');

I can access the same database (Oracle 8.1.6) without error from machines
with the same version of perl but earlier DBDs.

My NLS_CHARACTERSET is US7ASCII (from $dbh->ora_nls_parameters()).

Any guidance as to how to correct this error would be much appreciated.

TIA

Dan