use Spreadsheet::WriteExcel;

$workbook = Spreadsheet::WriteExcel->new('sample.xls');
$worksheet = $workbook->add_worksheet('ws name');
$worksheet->write(0,0,'something in a1');
$workbook->close();
[download]

use IO::File;
use Spreadsheet::WriteExcel;

my $infile = IO::File->new("<infile.csv");
my $workbook = Spreadsheet::WriteExcel->new('output.xls');
my$worksheet = $workbook->add_worksheet('ws name');

my $row = 0;
while (my $line = $infile->getline()) {
   chomp($line);
   my $record = [split ',', $line];
   $worksheet->write_row($row,0, $record);
   $row++;
}

$infile->close();
$workbook->close();
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  print "Content-type: application/vnd.ms-excel\n\n";
  print join("\t", @cols) . "\n";
  foreach my $row ( @rows ){
    print join("\t", @$row) . "\n";    # if rows are array refs
    print join("\t", @{$row}{@cols} ) . "\n"; # if rows are hash refs
  }
[download]

Outputting a file.csv and opening it in Excel works fine, but doesn't have that classic ".xls" extension.

Outputting a CSV file as file.xls doesn't work, (at least, with my version of office) but saving tab-delinated data as file.xls works fine.

However, both the above solutions above will cause a prompt if the user tries to save the file: "file.xls may contain features that are not compatiable..." This may be confusing if the user doesn't understand why they are getting the message.

I've worked with Spreadsheet::ParseExcel quite a bit, and if you need to output native excel, it's a pretty good module. You can add formatting like freezing the header row so it scrolls with the data, and fancy stuff like font formatting.

If you want to get really tricky, and you're comfortable with XML, create a spreadsheet with sample data and all the formatting and features you want, and save it as XML. Then write perl to substitute the sample data with the data you want to display. If you're careful to output the same tags that Excel used originally, you get a native-excel file with all the advanced features you could want.

Not suprisingly, Excel is pretty picky that you give it the XML format it expects, but with an existing file as a template, it's not that difficult.

Update:
My data is not comma delimited, it's tab delimited. I can't say if this works with comma delimited files (without lying, that is). Disregard my worthless advice.

D'oh! I posted from the preview-able window. Argh! What do you do when this happens?