Likewise, I cannot replicate on W2k re 11/1/2008 with Perl 5.8.8 BUT the problem is reproduced on Ubuntu 6.0.6 with Perl 5.8.7.

But using 10/24/2008 as the start day under W2k:

We expect a start day and a day 6 days later than that!
The start day is 10/24/2008 and the end day is 10/29/2008
[download]

while running under Ubuntu, the end day is returned as 10/30/2008.

Note that I took some minor liberties with your code: I hardcoded each $start_date and commented out the use of GetOpt::Long).