...
    my $typehint_raws = [ ($typehint_raw) x  @$arrayref];
    my $json = encode_json($arrayref, $typehint_raws);
    return $json;
    ...
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I'm looking at $typehint_raws just before calling the encode_json command.

If you're dumping $typehint_raws just before calling encode_json(), then you're dumping it just after initializing it (according to the posted code). In that case, the situation is just as given in the example here except that it's a hash reference that's being replicated rather than an array ref.

I don't know what encode_json() does to "fill in" $typehint_raws, but it doesn't seem that it can be of much consequence since $typehint_raws goes out of scope with the return statement that immediately follows the subroutine call.