If you want to use the pack geometry manager, you need to pack both the widget and it's container (if it isn't a top level) with -expand => 'true' (for any value of true) and -fill => 'both'.

use warnings;
use strict;
use Tk;

my $mw = Mainwindow->new;
my $frame = $mw->Frame->pack(-expand => 1, -fill => 'both');
my $text = $frame->Scrolled('Text',
     -scrollbars => 'se',
)->pack(-expand => 1, -fill => 'both');

MainLoop;
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