--- loncom/homework/math_parser/CalcEnv.pm	2015/06/29 15:42:13	1.1
+++ loncom/homework/math_parser/CalcEnv.pm	2015/06/30 17:42:14	1.2
@@ -36,9 +36,12 @@ use aliased 'Apache::math_parser::Units'
 # @param {boolean} unit_mode
 ##
 sub new {
-    my $class = shift;
+    my ($class, $unit_mode) = @_;
+    if (!defined $unit_mode) {
+        $unit_mode = 0;
+    }
     my $self = {
-        _unit_mode => shift // 0,
+        _unit_mode => $unit_mode,
     };
     if ($self->{_unit_mode}) {
         $self->{_units} = Units->new();