/usr/lib/x86_64-linux-gnu/libtcl.so is in tcl-dev 8.6.0+6ubuntu3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libtcl8.6.so
/usr/lib/x86_64-linux-gnu/libtcl8.6.so