set pipe [open |cat.exe r+]
fileevent $pipe readable .stderr.processing.
set process [open "|process.exe 2>@$pipe" r+]
fileevent $process readable .stdout.processing.
http://wiki.tcl.tk/880
Perl:TK - standard output to text widget
http://www.perlmonks.org/?node_id=332324
Saludos!!