use Win32::OLE::Const 'Microsoft Excel';
[...]
$WS->Cells($i,11)->FormatConditions ->Delete;
$WS->Cells($i,11)->FormatConditions->Add(xlExpression,,
   "=AND(J$i>0,K$i=0)");
$WS->Cells($i,11)->FormatConditions(1)->Interior->{ColorIndex}=
   xlYellow;
[...]