cron a script?

how would i cron a perl script to to chmod all php files to 755 or is there an easier way, could i just : crontab chmod *.php 755. would those work?

First run this:
man 5 crontab

This will show you the format of how to place commands in your crontab.

Next, to edit your crontab file, you need to do:
crontab -e

This will bring up your crontab file and you can edit it and add the commands that you like. Don't forget to specify absolute pathnames, as in:

/bin/chmod 755 /path/to/*.php