To edit the crontab type (it normally uses vi as editor)
crontab -e

To add a scheduled type
0 21 * * 1-7 perl /tmp/test.pl

To make sure the crontab is updated correctly restart it
/etc/init.d/cron restart

Explanation
0 Minutes (0-59)
21 Hour (0-23)
* Day (1-31)
* Month (1-12)
0-7 Weekday (0-7, Sunday = 0)

Exampels
*/5 * * * * “Runs every 5 minute, every day, all year”
*/5 1,10 1,6 * “Runs every 5 minutes, January and July 1 and 10”

Readmore