use openw_utf8 instead of filehandle method

The use of the filehandle method from Path::Tiny was incorrect, but it
was not caught until a later version of Path::Tiny. Use the openw_utf8
method instead.

The same fix was applied to WICS-GUI.pl in d4a7f58, but I forgot to fix
WICS.pl.
Merge branch 'master' of https://github.com/renatb/ITS2.0-WICS-converter