When you say it would be a waste of resources, are you sure that the resources would be anything more than trivial? Running a simple perl script via cron is not usually something that causes servers to grind to a halt with steam pouring out of them.
Also, if you're getting 600 users a day, then presumably they are arriving at (about) 25 per hour, so running the cron every ten minutes is pretty likely to catch new users. And because you can catch multiple new users at once, you might find that the script takes less load than calling once for every new user.
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