open my $fh, '-|', 'sort', $filename or die;
chomp(my $line = <$fh>);
my ($prev_key, $prev_value) = split /,,/, $line;
while (<$fh>) {
  chomp;
  my ($key, $value) = split /,,/;
  if ($prev_key eq $key) {
    $prev_value .= $value;
  }
  else {
    print "$prev_key,,$prev_value\n";
    ($prev_key, $prev_value) = ($key, $value);
  }
}
print "$prev_key,,$prev_value\n";