I am having trouble modifying a currently active excel window with Win32::OLE.

I want to be able to do the following:
1) Open an excel spreadsheet called 'SummarySheet.xls' manually.
2) THEN run a perl script that manipulates that spreadsheet.

This is no problem if I confine myself to modifying only the active sheet. This code, for instance, works fine:

use Win32::OLE::Variant;
use Win32::OLE qw(in with);
use Win32::OLE::Const "Microsoft Excel";
use strict;
my $excel = Win32::OLE->GetActiveObject('Excel.Application')||die "no"
+;
    $excel -> Windows("SummarySheet.xls") -> Activate;
    my $workbook = $excel    -> Activewindow;
    my $sheet    = $workbook -> Activesheet;
    $sheet -> Cells(1,1) -> {Value} = "This works fine.";
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However, when I attempt to choose the sheet to manipulate, I get an error saying that the sheet isn't defined:

use Win32::OLE::Variant;
use Win32::OLE qw(in with);
use Win32::OLE::Const "Microsoft Excel";
use strict;
my $excel = Win32::OLE->GetActiveObject('Excel.Application')||die "no"
+;
    $excel -> Windows("SummarySheet.xls") -> Activate;
    my $workbook = $excel    -> Activewindow;
    my $sheet    = $workbook -> Sheets(2);
    $sheet -> Cells(1,1) -> {Value} = "This does not work fine.";
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I would assume that somehow I'm using the wrong syntax to define the sheet, but if I create a new workbook (instead of selecting the existing one), it works fine:

my $excel = Win32::OLE->GetActiveObject('Excel.Application')||die "cou
+ldnt get active one";
     my $workbook = $excel    -> Workbooks -> Add;
     $workbook->Activate;
    my $sheet    = $workbook -> Sheets(2);
    $sheet -> Cells(1,1) -> {Value} = "This works well";
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To sum up, I manually open a spreadsheet called "SummarySheet.xls", and I am trying to work things so I can then run a perl script that will modify multiple sheets of that spreadsheet. Thanks in advance for your help.