...

$sth->execute(...) or die(...);

my %field_idx = do {
   my $i = 0;
   map { $_ => $i++ } @{$sth->{NAMES}}
};

my $row;
while ($row = $sth->fetch()) {
   print($row->[$field_idx{'userid'  }], $/);
   print($row->[$field_idx{'password'}], $/);
}

...