my $sql = "select * from x";
my $sth = $dbh->prepare($sql)->execute(); # fehlerabfangen vergessen...
while (my $row = $sth->fetchrow_hashref()) {
  $dbh->do("UPDATE x set y='z' WHERE id=" . $dbh->quote($row->{id})
    or die "Error in sql: $DBI::errstr\n";
} # while
$sth->finish();