TeX Live 2020 - tlmgr cannot load perl-tk although it is installed

I installed TeX Live 2020 on my Linux system. When I try to invoke tlmgr --gui I get the error message

tlmgr: Cannot load Tk, thus the GUI cannot be started!
The Perl/Tk module is not shipped with the TeX Live installation.
You have to install it to get the tlmgr GUI working.
(INC = /usr/local/texlive/2020/texmf-dist/scripts/texlive /usr/local/texlive/2020/tlpkg /home/robert/anaconda3/lib/site_perl/5.26.2/x86_64-linux-thread-multi /home/robert/anaconda3/lib/site_perl/5.26.2 /home/robert/anaconda3/lib/5.26.2/x86_64-linux-thread-multi /home/robert/anaconda3/lib/5.26.2 .)

See https://tug.org/texlive/distro.html#perltk for more details.
Goodbye.

However, I do have perl-tk installed. sudo apt-get install perl-tk returns ... perl-tk is already the newest version (1:804.033-2build1).

With TeX Live 2019, which I had installed previously on the same system, this problem did not occur.
Any idea how I could get tlmgr --gui to work?