The example code below makes perl 5.16 dump core, and 5.18 die with this error:
Can't call method "export" on an undefined value at /usr/lib/perl5/current/Exporter/Heavy.pm line 214.
BEGIN failed--compilation aborted at Utils.pm line 4.
Compilation failed in require at MainWindow.pm line 6.
BEGIN failed--compilation aborted at MainWindow.pm line 6.
Compilation failed in require at ./example.pl line 5.
BEGIN failed--compilation aborted at ./example.pl line 5.

If I use only object-oriented or only Exporter-based modules, then it works and does not crash. Mixing both types triggers the bug. Gtk2 version of the programs also works correctly in any case.

example.pl:
#!/usr/bin/perl -w

use strict;
use Gtk3 '-init';
use MainWindow;

my $window = MainWindow->new();
$window->show_all();
Gtk3->main();

MainWindow.pm:
package MainWindow;

use strict;
use Gtk3;
use parent -norequire, 'Gtk3::Window';
use Utils;


sub new
{
  my($class) = @_;
  my $self = bless Gtk3::Window->new('toplevel'), $class;
  $self->add(entry_with_clear_button());
  $self->set_title('Gtk3-Perl Bug Example');
  $self->signal_connect(delete_event => sub { Gtk3->main_quit(); });
  return $self;
}

1;

Utils.pm:
package Utils;

use strict;
use Gtk3;
use Exporter;
use vars qw(@ISA @EXPORT);

@ISA = qw(Exporter);
@EXPORT = qw(entry_with_clear_button);

sub entry_with_clear_button
{
  my $entry = Gtk3::Entry->new();
  $entry->set_icon_from_stock('secondary', 'gtk-clear');
  $entry->set_icon_activatable('secondary', 1);
  $entry->signal_connect(icon_release => \&clear_entry);
  return $entry;
}

sub clear_entry
{
  my($self, $pos, $event) = @_;
  if($pos eq 'secondary')
  {
    $self->set_text('');
  }
}

1;