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use Data::Dumper;
my $prev_data;
...
  }
  $prev_data = $data;
}

SELECT t.col0, t.col1, t.col2, t.col4
FROM mytable as t
...
    AND tmp.col1 = t.col1
    AND tmp.col2 = t.col2
    AND tmp.col4 <> t.col4