This is a variable found in Unix interactive tclsh.Detail on this page the purpose for which it is used.
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'''[DGP]''' 2003-05-14: tcl_libPath has only one documented use.
It holds a list of directories, and the C-level '''encoding''' documentation
says that if you add another directory to that list, then the
new directory will be searched for ''*.enc'' files that hold
additional encodings for Tcl to recognize.

But... that documentation is false.  This means of extending
the encoding search path does not work.  Never has.  See
Tcl Bug 463190.

So,... Tcl programmers should just pretend ''tcl_libPath''
does not exist.  It cannot be used for anything useful.
Treat it as an internal detail of Tcl, not for your use.