I read that Tk is on its 8.5 version. Why do I find only the 8.4 version with Perl (cpan, Activestate)?

Because Tk.pm is based on the 8.4 version only

I guess the closest thing to Perl/Tk is Tkx. Anyone experienced ith translating any big project? Is it a big pain?

No, the closest thing is Tcl::pTk, it depends on tix/ctext/plotchart for looks

Tkx is the worst possible option

Any alternatives to reduce the recoding time, still having a decent good looking app?

Use a browser as the GUI?