use Win32::OLE::Variant;
...
        (my $ls = $hashref->{'Last Saved'}) =~ s/^(\d+)-(\d+)-(\d+)T(\d+):(\d+):(\d+)$/$1 $2 $3 $4:$5:$6/;
        if ($ls ne "") {
            my $var = Variant(VT_DATE, $ls);
            $worksheet->Cells($row, $col++)->{'Value'} = $var;
        } else {
            $worksheet->Cells($row, $col++)->{'Value'} = '-';
        }