diff --git a/lib/Slic3r/GCode.pm b/lib/Slic3r/GCode.pm
index 5f75905dc..b895a1d99 100644
--- a/lib/Slic3r/GCode.pm
+++ b/lib/Slic3r/GCode.pm
@@ -188,6 +188,10 @@ sub extrude {
 sub extrude_loop {
     my ($self, $loop, $description) = @_;
     
+    # make a copy; don't modify the orientation of the original loop object otherwise
+    # next copies (if any) would not detect the correct orientation
+    $loop = $loop->clone;
+    
     # extrude all loops ccw
     my $was_clockwise = $loop->make_counter_clockwise;
     my $polygon = $loop->polygon;
diff --git a/t/perimeters.t b/t/perimeters.t
index 0805d1b42..526c7a5b9 100644
--- a/t/perimeters.t
+++ b/t/perimeters.t
@@ -43,7 +43,11 @@ use Slic3r::Test;
     
     {
         $config->set('external_perimeter_speed', 68);
-        my $print = Slic3r::Test::init_print('cube_with_hole', config => $config);
+        my $print = Slic3r::Test::init_print(
+            'cube_with_hole',
+            config      => $config,
+            duplicate   => 2,  # we test two copies to make sure ExtrusionLoop objects are not modified in-place (the second object would not detect cw loops and thus would calculate wrong inwards moves)
+        );
         my $has_cw_loops = my $has_outwards_move = 0;
         my $cur_loop;
         my %external_loops = ();  # print_z => count of external loops
@@ -58,6 +62,11 @@ use Slic3r::Test;
                     $has_cw_loops = 1 if Slic3r::Polygon->new_scale(@$cur_loop)->is_clockwise;
                     if ($self->F == $config->external_perimeter_speed*60) {
                         my $move_dest = Slic3r::Point->new_scale(@$info{qw(new_X new_Y)});
+                        
+                        # reset counter for second object
+                        $external_loops{$self->Z} = 0
+                            if defined($external_loops{$self->Z}) && $external_loops{$self->Z} == 2;
+                        
                         $external_loops{$self->Z}++;
                         my $loop_contains_point = Slic3r::Polygon->new_scale(@$cur_loop)->contains_point($move_dest);
                         $has_outwards_move = 1