lost number formats

[mw]

I am generating a spreadsheet with Perl. There are columns of numbers that I
have formatted, through code, as Custom #,###.00. When I open the
spreadsheet, using the Excel icon, I get the message: File error. Some
number formats may have been lost. All of the formatting is there - my
columns of numbers look like they should. What could be the problem, or how
can I prevent the error from appearing when the spreadsheet is opened, i.e.,
turn off the error.

Thanks.