What is wrong with this? ... This gives me a not initialized value.

http://search.cpan.org/perldoc/Win32#Win32::GetShortPathName(PATHNAME) documents the caveats pretty well -- your system probably doesn't support short names or the filename doesn't exist or something else ( $^E, %! )

...Tk GUI...

chooseDirectory doesn't give you unicode; Read Tk::chooseDirectory and Re: Perk Tk - how to send a value from a subroutine back to the main program using Button (without MainLoop) and especially http://search.cpan.org/perldoc/Tk::chooseDirectory#CAVEATS

Also read Win32::Unicode and use Win32::Unicode::Native