I use selectcol_arrayref all the time, and it works very well. Make sure you are checking the value of your $dbh->err, and $dbh->errstr because there may be an error in your query.

The difference between selectcol_arrayref and selectall_arrayref is that the former returns a single array of all values in that column, while the latter returns an array of arrays of each row (AoA), even if each row contains only one entry. This is why the former is preferable for these kinds of circumstances.