/usr/lib/tcl8.6/tclConfig.sh is in tcl8.6-dev 8.6.5+dfsg-2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
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#! /bin/sh . /usr/lib/`dpkg-architecture -qDEB_HOST_MULTIARCH`/tcl8.6/tclConfig.sh