You are absolutely right, it does. Thanks for pointing that out, I'd missed it (probably because there wasn't an example that used it). However, changing the code to:

my $new_table = XBase->create("name" => "$filename.$ext",
                                      "field_names" => \@column_names,
                                      "field_types" => undef,
                                      "field_lengths" => undef,
                                      "field_decimals" => undef);
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had no effect - I still get the same error.