Thanks, Bart, for your help. 'undef' in the second position is necessary. However, this method does something "relative", so I had to add dollar signs to the formula. Also, although I have 'use Win32::OLE::Const "Microsoft Excel";' in the code, xlYellow didn't work; I substituted '6'.

$WS->Cells($i,11)->FormatConditions->Add
  (xlExpression,undef,"=AND(\$J\$$i>0,\$K\$$i=0)");
$WS->Cells($i,11)->FormatConditions(1)->Interior->
  {ColorIndex} = 6; #xlYellow
[download]

Problem solved. Thanks for everyone who chimed in.