Another step towards C++ presets.

slic3r.pl

@@ -100,12 +100,17 @@ if ($opt{save}) {
 my $config = Slic3r::Config->new_from_defaults;
 $config->apply($cli_config);
 
+# locate or create data directory
+# Unix: ~/.Slic3r
+# Windows: "C:\Users\username\AppData\Roaming\Slic3r" or "C:\Documents and Settings\username\Application Data\Slic3r"
+# Mac: "~/Library/Application Support/Slic3r"
+Slic3r::set_data_dir($opt{datadir} || Wx::StandardPaths::Get->GetUserDataDir);
+
 # launch GUI
 my $gui;
 if ((!@ARGV || $opt{gui}) && !$opt{save} && eval "require Slic3r::GUI; 1") {
     {
         no warnings 'once';
-        $Slic3r::GUI::datadir       = $opt{datadir} // '';
         $Slic3r::GUI::no_controller = $opt{no_controller};
         $Slic3r::GUI::no_plater     = $opt{no_plater};
         $Slic3r::GUI::autosave      = $opt{autosave};