It's slightly more complicated: you have to use the foreach method of the model to iterate on its rows, supply a callback to it, in which you use the Gtk2::TreeIter to get the value out of each row.

Working example:

#! /usr/bin/perl
use strict;
use warnings;
use feature qw/say/;
use Gtk2 -init;

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

my $vbox = Gtk2::VBox->new();
$window->set_size_request (130, 100);
$window->add($vbox);

my $combo = Gtk2::ComboBox->new_text();
foreach (qw/foo bar baz/) {
    $combo->append_text ($_);
}
$vbox->pack_start($combo, 0, 0, 5); # expand?, fill?, padding

my $button = Gtk2::Button->new ('extract');
$button->signal_connect (clicked => \&extract);
$vbox->pack_start($button, 1,1,5);

$window->show_all;
Gtk2->main;

sub extract {
    my $active_text = $combo->get_active_text();
    print "active text: $active_text\n";
    print "all rows:\n";
    my $model = $combo->get_model();
    $model->foreach(sub {
        my ($store, $path, $iter) = @_;
        my $value = $model->get_value($iter, 0); # assuming there is o
+ne column only in the model
        print $value."\n";
        return 0;
    });
}
[download]

Documentation: http://gtk2-perl.sourceforge.net/doc/pod/Gtk2/ComboBox.html - warning: it's autogenerated and obsolete.