I don't understand. Is this a math question?

15 * PI   = 47.12 (approximately)
47.12 / 180 = 0.26  (approximately)
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So one division for every 2.6mm will get you 180 divisions on a circle with a diameter of 15cm.

If you have a Perl question please feel free to post specifics on what part of the code you're having difficulty with.