''I just want to clear the screen'' sounds like such a simple thing.
Why is it difficult?  Because there are so many types of screens!  
Are we talking about a terminal emulator program, like xterm, rxvt, telnet?
Are we talking about a physical terminal on a serial line (vt220)?  Is
it a line printer???? Is it a Tk text widget?

Here are a few ways:

    exec clear >@ stdout        ; # Most unix systems should have a clear command

    puts \x1B\[2J               ; # for Solaris xterm - VERY terminal specific

    eval exec [auto_execok cls] ; # Most windows systems should have a cls

    exec command /c cls         ; # DOS?

    exec >&@stdout $::env(COMSPEC) /c cls    ; # Win95

    $t delete 1.0 end           ;# Tk text widget

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[Bruce Hartweg] suggests [http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&th=57f93208a1685681]
    eval exec >&@stdout <@stdin [auto_execok cls]

''Pretty neat tricks ;)'' -- [Ro]

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'''Angel Sosa''':
In addition the following examples clears the whole screen 
''(on which platforms?)''

  puts \x1b\[H\x1b\[2J

This initializes and clears the screen:
  exec tput clear >@ stdout

  puts [exec clear]

These are pretty handy as well!!!!
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[[
[Category Text Screen] |
[GSoC2007 Tcl/Tk Application]
]]