Free @_ before spawning a new thread (known old Perl bug) as an attempt to fix the unref scalars error

lib/Slic3r/GUI.pm

@@ -288,6 +288,7 @@ sub check_version {
     my %p = @_;
     Slic3r::debugf "Checking for updates...\n";
     
+    @_ = ();
     threads->create(sub {
         my $ua = LWP::UserAgent->new;
         $ua->timeout(10);

lib/Slic3r/GUI/Plater.pm

@@ -545,6 +545,7 @@ sub export_gcode {
     
     $self->statusbar->StartBusy;
     if ($Slic3r::have_threads) {
+        @_ = ();
         $self->{export_thread} = threads->create(sub {
             $self->export_gcode2(
                 $print,
@@ -739,6 +740,7 @@ sub make_thumbnail {
         }
     };
     
+    @_ = ();
     $Slic3r::have_threads ? threads->create($cb)->detach : $cb->();
 }