-Mre=debug reveals the answer:

...
Compiling REx `\d{3-1}'
...
   1: DIGIT(2)
   2: EXACT <{3-1}>(5)
   5: END(0)
...
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Apparently this means the regex got compiled to match a digit, and then the literal text {3-1}. It would be nice if perl recognized that there might have been confusion with the {M,N} syntax, and thus would throw a warning, though issues of backwards compatiblity would need to be considered in doing this.