--tmpdir option has no effect as documented

my command line:
/usr/bin/innobackupex --user=root --password=xxxx --stream=xbstream /opt/tmp/ | nc otherhost port
And I ran 'nc -l port' on the other host to receive backup data.

I noticed that space of root filesystem decrease quickly. So I ran
lsof -c xtrabackup|grep delete
I get this:
xtrabacku 25266 root 5u REG 8,1 586478592 1040448 /tmp/xtrabackup_logfileIrqJoG (deleted)
And, innobackupex said:
130128 14:11:06 innobackupex: Starting ibbackup with command: xtrabackup_51 --defaults-group="mysqld" --backup --suspend-at-end --target-dir=/tmp --stream=xbstream

I read the document and set --tmpdir=/opt/tmp option to enforce it write files into /opt filesystem ( actually the document said a path after --stream would be used as --tmpdir ), but whether I set --tmpdir or not, it still write temporary file in /tmp which is a part of root filesystem.
At last, I modified /etc/my.cnf and set "tmpdir=/opt/tmp" in it, innobackupex writes temporary files in the path I want after the modification.

I use CentOS 5 and below is the version of percona-xtrabackup:
$ rpm -qi percona-xtrabackup
Name : percona-xtrabackup Relocations: (not relocatable)
Version : 2.0.5 Vendor: (none)
Release : 499.rhel5 Build Date: Fri 18 Jan 2013 04:29:13 AM CST
Install Date: Mon 28 Jan 2013 01:44:41 PM CST Build Host: localhost.localdomain
Group : Server/Databases Source RPM: percona-xtrabackup-2.0.5-499.rhel5.src.rpm
Size : 11344684 License: GPLv2
Signature : DSA/SHA1, Fri 18 Jan 2013 08:13:26 AM CST, Key ID 1c4cbdcdcd2efd2a
Packager : Percona Development Team <email address hidden>
URL : http://www.percona.com/software/percona-xtrabackup/
Summary : XtraBackup online backup for MySQL / InnoDB
Description :
Percona XtraBackup is OpenSource online (non-blockable) backup solution for InnoDB and XtraDB engines.

btw, I tried to debug the perl script, but not good at perl.
I commented the line 1728~1730 out, like below:
   1728 #if(!$option_tmpdir) {
   1729 # $option_tmpdir = get_option(\%config, $option_defaults_group, 'tmpdir');
   1730 #}
the script said:
130128 14:14:41 innobackupex: Starting ibbackup with command: xtrabackup_51 --defaults-group="mysqld" --backup --suspend-at-end --target-dir= --stream=xbstream
the --target-dir become empty!! There would be some error in innobackupex script that cause the $option_tmpdir variable lost.