my $csv = Text::CSV->new ({ binary => 1, auto_diag => 1, eol => $/ });
open my $fh, ">:encoding(utf8)", $filename or die "$filename: $!";

$sth = $dbh->prepare ($query) or # No need to put "'s around $query
    error ($q, "Problem with database call");
if ($sth->execute) { # not using RaiseError?
    while (my $ref = $sth->fetch) { # fetching data by ref is faster
        $csv->print ($fh, $ref);
        }
    }