for(my ($i,$j) = (2,1);$i<=$size+2;$i++,$j++){
my $formula = $worksheet->write('D'.$i,"=IF((C${j}-C${i})*6 < 0, 0,(C${j}-C${i})*6)");        }

"=IF((C${j}-C${i})*6 < 0, 0,(C${j}-C${i})*6)"