Short intro to cron. Call cron with this on the command line (as the user who should run the script):

export EDITOR=emacs
crontab -e
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Substitute 'emacs' with your favourite editor if you like

Then your editor should start and you can input a line

0 0 * * *        script.pl
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The first 0 means minute, the second hour. The next fields are day of month, month and day of week, and a '*' is equivalent to 'any'. So it is 0:00 of any day in any month and any day of the week.

Save and from then on cron will call your script.pl every night at 12