You are close, your use of packForget is wrong. There is no pack('forget'). Also you need to watch your memory when destroying widgets, Tk has a nasty habit of leaving refcounts around which cause memory gains over time. So try to reuse the $tx variable. Here is how I would do it. Don't destroy the Frame, as that causes an unpleasant window size change.

#!/usr/bin/perl
use warnings;
use strict;
use Tk;

my $mw = MainWindow->new;

my $mainFrame = $mw->Frame()->pack(-side=>'top');

my $tx; # reuse global to prevent memory gains
rebuild(); #setup first use

# script is called, runs, outputs to Scrolled above, user 
# chooses new option and sub clear() is called:
my $button = $mw->Button(-text=> 'Clear',
             -command => \&clear
   )->pack();

my $button1 = $mw->Button(-text=> 'Rebuild',
             -command => \&rebuild
   )->pack();


MainLoop;

sub clear{
       #$mainFrame ->destroy(); # don't, causes screen jitter
       $tx->packForget;
       # $tx ->pack('forget'); #wrong usage 
       
           #clean out a top level
#    my @w = $tl->packSlaves;
#    foreach (@w) { $_->packForget; }
       
}

sub rebuild{

$tx = $mainFrame->Scrolled(qw/Text 
    -font normal
        -width 73
    -height 15
    -wrap word 
    -scrollbars e/);
$tx->pack;

tie *STDOUT, 'Tk::Text', $tx;

print "foobar!".time."\n";
}
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