G'day Hugo,

I would've implemented the layout you describe as follows.

#!/usr/bin/env perl

use strict;
use warnings;

use Tk;

my $mw = MainWindow::->new();

my $fAB = $mw->Frame()->pack(-fill => 'both', -expand => 1);

my $fA = $fAB->Frame(-bg => '#ff0000'
)->pack(-side => 'left', -fill => 'both', -expand => 1);
my $fB = $fAB->Frame(-bg => '#ffff00'
)->pack(-side => 'left', -fill => 'y');
my $fC = $mw->Frame(-bg => '#0000ff'
)->pack(-fill => 'x');

$fA->Label(-text => 'A')->pack();
$fB->Label(-text => 'B')->pack();
$fC->Label(-text => 'C')->pack();

MainLoop;
[download]

I've colour-coded the A,B and C frames so that the effects of resizing are more obvious.