Add /i and I get a more verbose (and non-fatal) error only if "use encoding" is commented out:

Malformed UTF-8 character (unexpected continuation byte 0xb6, with no 
+preceding
start byte) in pattern match (m//)
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Which indicates that the regex engine is starting a step at the second byte of the multi-byte character.

And the ways that the error comes and goes for nonsensical changes makes me suspect there might be something like alignment or buffer overflow involved.

(Updated.)