Alternately, you could use Excel::Template, which uses Spreadsheet::WriteExcel.

use Excel::Template;

my $template = Excel::Template->new(
    filename => 'template.xml',
);

$template->param(
    hello => 'Hi Excel!',
    value => 1.2345,
    formula => '=SIN(PI()/4)',
);

$template->write_file('perl.xls');

--------
template.xml
--------
<workbook>
  <worksheet>
    <row>
      <bold>
        <cell><var name="hello" /></cell>
      </bold>
      <cell col="+1" text="$value" />
      <formula><var name="formula" />
    </row>

    <cell row="1" col="0"><var name="hello" /></cell>
  </worksheet>
</workbook>
[download]

I don't have all the formatting options built in, yet, but the rest of it works. It takes the same data structure as HTML::Template, so it's really good for building reports. (My current job has me using HTML::Template, Excel::Template, PDF::Template, and Graph::Template for this very purpose.)