if ($query->param('excel')){
  &myexcelsubroutine
}

sub myexcelsubroutine {
  my $application = Win32::OLE->new("Excel.Application");
  my $workbook    = $application->Workbooks->Add;
  $application->{Visible} = 1;
  my $worksheet   = $workbook->Worksheets(1);
  etc etc
}