As others stated this seems to be a fix limit. However that buggered my programs up only when some sheet was spread onto two and the normally fixed count of worksheet contents was messed up. This is a pain but it may help that you can use as many spreadsheets as you like (well at least I am sure more than 106) but programming the graphics will be a real pain then if possible at all.

As for practical reasons if you do not need to apply any formatting in Excel later this Solution might work:
I only move data out of Excel. The Spreadsheet::ParseExcel and for file sizes that do not expand beyond sensible memory limits (20MB Excel file easily expand to 1GB of RAM) and have useable formatting. Otherwise use the OLE to get data out of Excel, which proves dead slow. Data is stored to XML, csv or sqlite. Graphics are done preferably with the Postscript::Simple module