Write a tab delimited file and name it .xls. Excel will know what to do with it.

If you want to get fancy, you can use [link|http://search.cpan.org/~jmcnamara/Spreadsheet-WriteExcel-2.14/lib/Spreadsheet/WriteExcel.pm|Spreadsheet::WriteExcel] n Perl to create a binary Excel file with whatever you want.

Cheers,
Ben