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DELETE FROM source dup
WHERE EXISTS (
  SELECT NULL
...
  WHERE dup.field1=source.field1
    AND dup.field2=source.field2
    AND source.key<dup.key)

my $data=$dbh->prepare("SELECT key, field1, field2 FROM source ORDER B
+Y field1, field2");
my $del=$dbh->prepare("DELETE FROM source WHERE key=?");
my @last;
...
}
$data->finish();
$del->finish();