[PT] posted this in [the comp.lang.tcl newsgroup]:
    package require Tk
    text .t -yscrollcommand {.s set}
    scrollbar .s -command {.t yview}
    pack .s -side right -fill y
    pack .t -side left -fill both -expand 1

    set f [open $filename r]
    set data [read $f]
    close $f

    .t delete 0.0 end
    .t insert 0.0 $data

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And [Marty Backe] posted this as a follow-up:

  package require Iwidgets
  iwidgets::scrolledtext .t
  pack .t -fill both -expand true

  .t insert $filename end


To clear the widget between file reads:

  .t clear