It appears to only happen when the $key is set at the 2nd value in the excel call (the one for the column). You set the key to the first value (the row), you need to set the key to the column value. I ran your code and it worked fine, but when I modified it so it was like my code (which crashed) I got the same error "Can't use an undefined value as a HASH reference at C:/test.pl line 22." 
use strict;
use warnings;
use diagnostics;
use Win32::OLE;

my $xl = Win32::OLE->new('Excel.Application');
$xl->{Visible} = 1;
my $wb = $xl->Workbooks->Add();
for (2..$wb->Sheets->{Count}) {
    $wb->Sheets(2)->Delete;
}

my $worksheet = $wb->Sheets(1);
my %hash = (1 => 'One', 
            5 => 'Five',
            10=> 'Ten');
            
my $row = 0;
foreach my $key (sort keys %hash){
    $row = $row + 1;
    print "Key: $key\nValue: $hash{$key}\n";
    $worksheet->Cells($row, $key)->{Value} = $hash{$key};
}

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In reply to Re^2: Can't use an undefined value as a hash reference when passing hash key into Excel OLE call. by kgnickl
in thread Can't Use an undefined value as a HASH reference when passing HASH key into Excel OLE call. by kgnickl

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