....

$Sheet->Range("A1")->{Value} = '$5.00';
$Sheet->Range("B1")->{Value} = '$-3.51';
$Sheet->Range("C1")->{Value} = '$-0.01';

my $selection = $Sheet->Range("A1:C1");

$selection->{FormatConditions}->Delete;
$selection->{FormatConditions}->Add(xlCellValue, xlLess, "0");
$selection->FormatConditions(1)->Font->{ColorIndex} = 3;

...