From Identifier parsing:

If working under the effect of the use utf8; pragma, the following rules apply:

/ (?[ ( \p{Word} & \p{XID_Start} ) + [_] ])
        (?[ ( \p{Word} & \p{XID_Continue} ) ]) *    /x
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And looking at the Unicode Character Properties of U+00B2 SUPERSCRIPT TWO, it's neither XID_Start nor XID_Continue. Although I'm not an expert on Unicode identifiers, this seems like it explains it.