#!/usr/bin/perl -w

use strict;
use Gtk2 -init;
use Glib qw /TRUE FALSE/;

my $vbox = Gtk2::VBox->new(FALSE);
my $small = Gtk2::Label->new('small phrase');
my $large = Gtk2::Label->new('really loooooooooooong phrase');

$small->set_justify('left'); 
$large->set_justify('left');

#What you need is 
$small->set_alignment(0, 0.5);

# The first argument controls horizontal alignment, 
# and 0 makes it align to the right. The second specified 
# vertical alignment, and 0.5 makes it 
# vertically centered. 0 will make it align to the top.

# to clarify --- GtkLabel isa GtkMisc, 
# and Gtk2::Misc::set_alignment() controls where within the 
# widget's allocation the contents are drawn.  
# Gtk2::Label;:set_justify() controls justification of 
# text in multiline label layouts.

$vbox->pack_start($small, FALSE, FALSE, 0);
$vbox->pack_start($large, FALSE, FALSE, 0);

my $window = Gtk2::Window->new;
$window->signal_connect(destroy => sub { Gtk2->main_quit; });

$window->add($vbox);

$window->set_default_size(200,50);
$window->show_all;
Gtk2->main;