If you are the one doing the "printing" and that's what it sounds like:

# make a frame, say frame_text, then create a text
# widget:

my $text = $frame_text->Scrolled ('Text', -scrollbars=>'se', )
                             ->pack(-expand=>1, -fill=>'both');
#append lines with:
$text->insert ('end', $_);

#delete all lines
$text->delete("1.0", 'end');
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Multi-threads and re-directing output from other program is more complex, but if all you need is that your "command line output" gets put into the GUI, just follow the above.