my $row = 1;
for my $filename (sort keys %error_per_file){
    $worksheet->write($row, 0, $filename)
    my ($errnum, $occurences);
    $worksheet->write($row, $column_for{$errnum}, $occurences)
        while ($errnum, $occurences) = each %{$error_per_file{$filename}};
    ++$row;
}