odd.

But please discover your $fileName variable.

Of course there should be no protection error.
May be backslashes backslashed once again in Tcl interpreter?
You may need

$interp->Eval("source {$fileName}");
[download]

Note curly braces,.
If there will be still problems, then show output of $interp->Eval("puts {$fileName}");

Courage, the Cowardly Dog