my $DB1 = DBI->connect($DSN,$UID,$PWD);
my $DB2 = DBI->connect($DSN,$UID,$PWD);
...
my $ST1 = $DB1->prepare("select ID from FOO");
my $ST2 = $DB2->prepare("update FOO set Bar=? where ID=?");
$ST1->execute();
while (my @row = $ST1->fetchrow_array) {
   $ST2->execute(++$cnt, $row[0]);
}