That has a drawback when stringification is overloaded. The logical next step is to take that into account:

sub reftype {
    require overload;
    local $_ = overload::AddrRef(shift);
    return /=(\w+(?:::\w+)*)\(0x[0-9a-f]\)\z/;
}
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I think you can put things into the package tables at runtime that won't match \w+(?:::\w+)*, but I can't imagine any sane use of that so I'd document it as a limitation and move on.

Update: substituted a star for the plus quantifier. Thanks theorbtwo.

Makeshifts last the longest.