my $found;
while (@row = $query->fetchrow_array) {
  ...
  if (...found desired data...) {
    $found = 1;
    # might also exit loop here with 'last'
  }
}
if ($found) {
  # data was found
} else {
  # data was not found
}

##</code><code>##

my $sql = "SELECT * FROM $course WHERE column = ?";
my $sth = $dbh->prepare($sql);
$sth->execute($userid);
if (@row = $sth->fetchrow_array) {
  # row was found where column == $userid
} else {
  # no such row found
}