Are the files big? If not, say each file <50M, I would read all the excel files using Spreadsheet::ParseExcel and create some dbm-like database (perl hash on disk) with some extra fields to later ease up your task of summarizing the data.

For this simple "database" the modules AnyDBM_File, MLDBM or DBM::Deep come to mind or even simple serializing to a file using YAML o Data::Dumper-like modules.

Maybe you can use a common origin in days to facilitate the drawing of a chart or table so that a pre-calculated record could be: issue, date0, date1, date1 - date0 (in days), day0, day1 (day0 and day1 taken with respect to an arbitrary origin -- easy to do with Date::Manip or Date::Calc). For the drawing I would use one of the Chart::* modules or gnuplot.