You example is a little confusing because you use the word "row" when it is a column. So I am assuming when you issue the select you have a problem with one of the columns is returned as undef and you don't expect it to be.

Try running the smallest example of what you are doing but with the environment variable DBI_TRACE set like "export DBI_TRACE=DBD=x.log" and show me the log output (in x.log) after the prepare and up to the end of the fetching.