I've installed Tk with cpan

Why? there's a prebuilt perl-tk package in the ubuntu repository. As for Ubuntu 16.04.3 LTS:

qwurx [shmem] ~> dpkg -l perl-tk
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/T
+rig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name              Version       Architecture  Description
+++-=================-=============-=============-====================
+====================
ii  perl-tk           1:804.033-1bu amd64         Perl module providin
+g the Tk graphics li
[download]

Since you are using the system perl, you should use the packages provided by your systems package manager

user@system:~$ sudo apt-get install perl-tk
[download]

and install via cpan/cpanm only if you absolutely need a different version.