Hello poopay, and welcome to the Monastery!

As explained in Excel::Writer::XLSX#DATES_AND_TIME_IN_EXCEL, Excel dates are stored internally as real numbers, but Spreadsheet::BasicRead and its relatives read dates in as strings. This means you have to manually extract the date from your input data, then convert it to a more convenient form. Here is one way to do this (adapted from the example given in the module documentation just referenced):

#! perl
use strict;
use warnings;
use Excel::Writer::XLSX;
use Spreadsheet::BasicRead;

my $workbook  = Excel::Writer::XLSX->new('perl.xlsx') or die "$!";
my $worksheet = $workbook->add_worksheet(); 
my @files     = ('data.xlsx');

foreach my $file (@files)
{
    my $excel = new Spreadsheet::BasicRead($file) or die "$!";
    my $row   = 0;

    while (my $dataref = $excel->getNextRow())
    {
        foreach my $date (@$dataref[7 .. 8])  # Columns H and I
        {
            $date =~ s[ ^ (\d{1,2}) / (\d{1,2}) / (\d{4}) $  ]
                      [ sprintf("%4d-%02d-%02d", $3, $2, $1) ]ex;
        }

        $worksheet->write_row($row++, 0, $dataref);
    }
}
[download]

Note: this approach will work only if you know the format(s) of the dates in your input files — or if you are prepared to expand the regex to allow for all the date formats you may encounter.

Update 1: I tested the above on an input file data.xlxs with Row 1 populated as follows:

1.1    2.1    3.3    4.4    5.5    6.6    7.7    9/15/2012    7/18/196
+0    8.8    9.9
[download]

The output file, perl.xlxs, had as Row 1:

1.1    2.1    3.3    4.4    5.5    6.6    7.7    2012-15-09    1960-18
+-07    8.8    9.9
[download]

as required.

Hope that helps,

Update 2: Why did you have a do {} block in your code snippet? As far as I can see, it serves no purpose there.

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