script works manually but not in cron job

I create my system graphs in rrd using perl script. When I run the script manually it'll update the graphs. On the other hand cron shows in logs that the job has been executed but it does not update graphs.. Any help please?

I've set apache2 directory permission root:apache with 770

Distro: CentOs 5.5.

Cron Config:

0,5,10,15,20,25,30,35,40,45,50,55 * * * *       /home/user/graphs/script-rrd.pl  > /dev/null 2>&1

I've already check /var/log/cron it shows that the above script has been executed but it does not update graphs in /var/html/www/graphs.png

Resolution of this issue:

cp -rf /opt/rrdtool-1.4.4/lib/perl/5.8.8/i386-linux-thread-multi/* /usr/lib/perl/site_perl

after copying rrd stuff in /usr/lib/perl5/site_perl issue has been resolved.

I've already created PERL5LIB. After creating PERL5LIB i was able to run scripts manually but it was not being run by cron so that is why i've to copied files as the above.

SELINUX is as following:
SELINUX=disabled
SELINUXTYPE=targeted

SETLOCALDEFS=0