WriteExcel->sheets() fails in getting worksheets

I am trying to read an excel file usign the writeexcel. Following is
the code-snippet:
my $workbook = Spread***::WriteExcel->new('Temp.xls');

print "Cannot create Temp.xls: $!\n" if (not defined $workbook);

foreach $worksheet ($workbook->sheets()) {
print "Work*** Name is: ".$work***->get_name()."\n";
}

The file Temp.xls is already existing in the directory. There are four
distinct worksheets in the workbook. I want to further modify the
worksheets. However, the sheets() method itself is not working.
What might be the reason? I'm using perl 5.8

Thanks,
Anand.
.