Readd use strict and use warnings

lib/Slic3r/GCode/PlaceholderParser.pm

@@ -1,4 +1,6 @@
 package Slic3r::GCode::PlaceholderParser;
+use strict;
+use warnings;
 
 sub new {
     # TODO: move this code to C++ constructor, remove this method

lib/Slic3r/Layer.pm

@@ -1,4 +1,6 @@
 package Slic3r::Layer;
+use strict;
+use warnings;
 
 use List::Util qw(first);
 use Slic3r::Geometry qw(scale chained_path);

lib/Slic3r/Layer/Region.pm

@@ -1,4 +1,6 @@
 package Slic3r::Layer::Region;
+use strict;
+use warnings;
 
 use List::Util qw(sum first);
 use Slic3r::ExtrusionLoop ':roles';