It looks like a bug in Object::MultiType. $last_value is an XML::Smart object (which derives from Object::MultiType) and overloads subtraction. Looks like line 168 ($n -= $_[0] ;) in the sub _OVER_deinc in multitype.pm may have it backwards. Changing the last two line of the sub to:

  my $temp = scalar $_[0];
  return $temp - $n;
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fixes the problem for this case. I can't guess what situation it may fail for though.

You should raise a bug tag against the module perhaps?

You can do that at rt://Object::MultiType