I strongly feel that Tkx is the way forward in GUI programming in Perl. It provides direct access to all widgets in Tck/Tk that look great (via the Tcl/Tk styles widgets), and can be written in a code that is similar enough to Perl Tk to make the conversion relatively simple.
However, the documentation for Tkx is not good and you will spend a lot of time trying to figure out the conversion from Tcl/Tk code to Perl Tkx in some cases. The conversion is simple but just isn't documented well.
There is good news though. Documentation at www.tkdocs.com/tutorial/ has recently been written that is clear and detailed. Anyone interested in Tkx should visit here before doing any serious work.
Perl Tk is certainly outdated. However, with good documentation via books like Mastering Perl it is relatively easy to create a simple multi-platform application. Porting to Mac, Linux, and Windows is also trivial via tools like ActiveState's PDK toolkit PerlApp program. However, without a lot of trial and error, it is difficult to get anything like an attractive looking program.
The Tk core appears to be no longer under development although users do contribute modules from time to time.
I am now switching to ActiveState's Tkx, which allows all recent Tck/Tk widgets to be used, and also seems to be just cleaner although the syntax is sometimes unintuitive. Converting a Tk app to Tkx is not very difficult either. Wx also seems to be gaining popularity as an alternative to Tk although there the perl specific documentation seems lacking.