I have to admit, I find this surprising. Looking at the perldocs for perlop, I see that:

        Left      Right      Description and pseudocode
        ==============================================================
+=
[...]
        Any       ARRAY      smartmatch each ARRAY element[3]
                       like: grep { Any ~~ $_ } ARRAY
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Ok, that's fine. So now I need to check how Any ~~ $_ works. So, a little further down, I see:

        Left      Right      Description and pseudocode
        ==============================================================
+=
[...]
        Any       Num        numeric equality
                        like: Any == Num
        Num       nummy[4]    numeric equality
                        like: Num == nummy
        undef     Any        check whether undefined
                        like: !defined(Any)
        Any       Any        string equality
                        like: Any eq Any
[download]

Now, this is interesting. When we see a number on the right, we compare with ==, even if the left doesn't look like a number. That seems wrong. I would have expected:

        nummy[4]  Num        numeric equality
                        like: nummy == Num
[download]

Maybe a bug report should be opened :-) (Though as I understand it, there's a fair bit of consternation about the whole smart-match thing in the first place.) I would also expect that nummy would include objects with == overridden.