Tcl::Tk has native widgets (unlike perl/Tk) so Tk-based interface generally are modern-looking, unless you design them to be old-fashioned.

See this example on having old-fashioned and modern widgets on same application, plus you can easily run Linux-looking widgets on win32, for example

See also recent meditaion PPM - It's a GUI!! - it deals with PPM GUI written in Tk but its very nice looking!