To have your trusty cron job only email errors, simply add this designation to the end of your cron entry:
*/30 * * * * /usr/local/bin/myscript.pl >/dev/null
Optional more geeky details:
There are two different kinds of output from any given command.
“>” is the same as issuing “1>” which is the non-error output. This is being sent to /dev/null in the above example.
“2>” is the “error” output. Leaving this out of your crontab entry as above means it is processed as normal (2> output is emailed to you).