it was easier for me to find in docs: I remember I saw it somewhere in Tk but had only Tcl/Tk handy.

In my opinion Tcl::Tk is more Tk than Tk, because first one refers to most recent and up-to-date one.

addition: as long as Tcl::Tk many compatible with Tk, and based on your post, here are two other ways which indeed work:

perl -MTcl::Tk -we "print Tcl::Tk::tkinit->server"
[download]

or

perl -MTcl::Tk=:perlTk -we "print tkinit->server"
[download]