If you are doing a SELECT COUNT(*), why are you using the rows() method. Just look at the query results.

UPDATE: Added code

my @row = $Statement->fetchrow_array;
if ($row[0] > 0)
{
    LogError("NICK_TAKEN");
    return("-1");
}
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Also, rows() doesn't always work when used with select statements.
From CPAN...

  $rv = $sth->rows;
    Returns the number of rows affected by the last row affecting comm
+and, or -1 if the number of rows is not known or not available.

    Generally, you can only rely on a row count after a non-SELECT exe
+cute (for some specific operations like UPDATE and DELETE), or after 
+fetching all the rows of a SELECT statement.

    For SELECT statements, it is generally not possible to know how ma
+ny rows will be returned except by fetching them all. Some drivers wi
+ll return the number of rows the application has fetched so far, but 
+others may return -1 until all rows have been fetched. So use of the 
+rows method or $DBI::rows with SELECT statements is not recommended.

    One alternative method to get a row count for a SELECT is to execu
+te a "SELECT COUNT(*) FROM ..." SQL statement with the same "..." as 
+your query and then fetch the row count from that.
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