you call $dbh->prepare:
  SQLPrepare(select whatever from whatever);

  you call $sth->execute
  SQLDescribeParam(called per parameter and parameters bound with SQLBindParameter) - does not happen in your case as you haven't got any
  SQLExecute
  SQLNumResultCols(to find if there is a result-set and how many columns)
  various calls to SQLDescribeCol and SQLColAttributes to find out about the columns

  you call $sth->fetchrow;
  If the columns are not already bound:
    SQLBindCol for each column
  SQLFetch (one row requested from ODBC driver)

  you call $sth->fetchrow;
  SQLFetch (one row requested from ODBC driver)