As Roddie pointed out in his comment, in some cases tclsh writes its output to the first line (usually console), not to the current line as expected. This “feature” has been introduced in IOS release 12.4(6)T1 (12.4(6)T is still OK) and fixed sometime before IOS release 12.4(11)T; 12.4(9)T is still broken (and I haven't tried all the other interim rebuilds ... or shall I call them rebreaks?).
Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.