Tk::DragDrop dynamically loads some modules at runtime which are determined by which OS the script is running under. PAR is probably not following the dependency chain far enough since the specific dependency is only know at run time.

Try manually adding the modules pathto/Perl/site/lib/Tk/DragDrop/Win32Site.pm and pathto/Perl/site/lib/Tk/DragDrop/Win32Drop.pm to the PAR build. (And possibly pathto/Perl/site/lib/Tk/DragDrop/Common.pm)