This was, and maybe is, the rule:
Postage stamps may be marked with perforation holes. The mark may not cover more than 1/4 square inch (1/2 inch by 1/2 inch) of the stamp, and the diameter of the holes may not exceed 1/32 inch.
Here is an archived USPS document with the rule:https://pe.usps.com/Archive/PDF/DMM...810/P022.pdf
Page 1, section 1.5
I tried two search strings to try to find a (confirmed) current document, and gave up with meaningless results...if someone has some time and a bit of Google-fu, perhaps that language is enough to dig it up...