[perl #125402] Assigning an Int to a substr-rw doesn't work

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# New Ticket Created by  Rob Hoelz 
# Please include the string:  [perl #125402]
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The following fails with "This type cannot unbox to a native string":

    my $s = 'foobar';
    $s.substr-rw(3, 3) = 1;

I would expect that substr-rw would DWIM and stringify the RHS of the 
assignment above.

test.p6
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