inferior-tcl-source-command

inferior-tcl-source-command is a variable defined in `tcl.el'.
Its value is

"source %s
"

• This variable may be risky if used as a file-local variable.

Documentation:
Format-string for building a Tcl command to load a file.
This format string should use `%s' to substitute a file name
and should result in a Tcl expression that will command the
inferior Tcl to load that file. The filename will be appropriately
quoted for Tcl.

You can customize this variable.