Hi, Try this,

$Sheet->Cells(3,1)->{Value} = "=IF(A1=B1,\"PASS\",\"FAIL\")";
[download]

Untested

Updated Tested code as

    use Win32::OLE;
    use strict;
    use warnings;

     my $ex;
    eval {$ex = Win32::OLE->GetActiveObject('Excel.Application')};
    die "Excel not installed" if $@;
    unless (defined $ex) {
        $ex = Win32::OLE->new('Excel.Application', sub {$_[0]->Quit;})
            or die "Oops, cannot start Excel";
    }
 
    my $book = $ex->Workbooks->Add;                
    my $sheet = $book->Worksheets(1);
    $sheet->Cells(1,1)->{Value} = "foo";    
   $sheet->Cells(1,2)->{Value} = "foo";    
  
   $sheet->Cells(3,1)->{Value} = "=IF(A1=B1,\"PASS\",\"FAIL\")";      
+ 
    
   $book->SaveAs( 'test.xls' );
     
__END__
foo    foo

PASS
[download]

I am not familiar with Excel. So i have just created a new excel sheet.