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

##</code><code>##

my $data=$dbh->prepare("SELECT key, field1, field2 FROM source ORDER BY field1, field2");
my $del=$dbh->prepare("DELETE FROM source WHERE key=?");
my @last;
while (my @record=$data->fetchrow_array()) {
  if ($record[1] eq $last[1] &&
      $record[2] eq $last[2]) {
    $del->execute($record[0]);
  }
  @last=@record;
}
$data->finish();
$del->finish();