What Platform are you on?
If you are actually on a Win32 Platform, just use Win32::OLE;

Here's a little example that I wrote for someone else showing how to get the contents of a cell, which, in this case, constitutes a date, and an example of manipulating said date using '::Variant'.

use strict;
use Win32::OLE qw(in with);
use Win32::OLE::Const 'Microsoft Excel';
use Win32::OLE::Variant;
use Win32::OLE::NLS qw(:LOCALE :DATE);

$Win32::OLE::Warn = 2; # Throw Errors, I'll catch them

my $Excel = Win32::OLE->GetActiveObject('Excel.Application')
        || Win32::OLE->new('Excel.Application', 'Quit');

    my $file = 'c:\perl\projects\excel\variant.xls';
    my $Book = $Excel->Workbooks->Open($file);
    my $Sheet = $Book->Worksheets("Sheet1");
    

my $dt = Variant(VT_DATE, $Sheet->Range("a1")->{Value});

print "$dt\n";

print $dt->Date(DATE_LONGDATE), "\n";
$Sheet->Range("a1")->{Value} = $dt->Date(DATE_LONGDATE);

print $dt->Date("ddd',' MMM dd yy"), "\n";
$Sheet->Range("a1")->{Value} = $dt->Date("ddd',' MMM dd yy");

$Book->ActiveSheet->Save();
[download]

Of course, if you aren't on Win32, the suggestions above hold much more merit than mine.

Update: I added code for writing out to the excel file, as that was part of the question.

C-.