undef does indeed invoke garbage destruction as can be shown in the following code.

#!/usr/bin/perl -w
use strict;

my $foo = new stuff;
my $bar = new stuff;
print $foo,"\n";
$foo = undef;
print STDERR "foo gone\n";
exit;

package stuff;
sub new
{
  my $self = [];
  bless $self;
  return $self;
}

sub DESTROY
{
  my ($self) = @_;
  print STDERR "destroying...\n";
}

1;
[download]

The output is:

stuff=ARRAY(0x80cb9c8)
destroying...
foo gone
destroying...

As you can see:

1. $foo gets undef'ed
2. DESTROY gets called for $foo
3. program exits, begins garbage collection
4. DESTROY gets called for $bar
5. program exits

The real question is do Gtk::CList objects get refrenced anywhere else or have circluar data structures that will prevent them from getting destroyed with the undef?