The statement may be true for Client-side applications that are supposed to run inside a webbrowser. Hasn't anyone heard that Perl has Tk, wxPerl, Win32::GUI or Gtk?
Is it badly documented? I think not. Tk is very well documented, there are even O'Reilly books about it. wxPerl has a very extensive documentation set, which is basically the wxWidgets documentation with wxPerl sidenotes.
If you can create advanced server-side applications with Perl, you can create advanced client-side applications too, just by adding a GUI to it, just like Java does
And if the problem is the Perl installation that 'needs' to be on the client's machine...just use PAR