You could do it in two steps -

Step 1 Explicitly get the row count with SQL -

SELECT count(*) FROM table where ...
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And then retrieve the rowcount from the query result.

Step 2 Prepare the actual SQL to fetch the data. Since you already know the row count from the previous step, you can easily implement the required logic.

$sth = $dbh->prepare($sql);
$sth->execute();
if ($rowcount == 1) {
  # do single row logic
  # fetch data
} else {
  # do multi-row logic
  # fetch data with while loop
}
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There is a rowcount variable in the DBI, but I would not rely on it because it is not reliable (and database dependent?).