I second rhesas recommendation for MooseX::AttributeHelpers, but for complete-ness sake I will also point out Moose::Autobox with which you could do this:

Package Wibble;
use Moose;
has 'thingies' => ( is => 'rw', isa => 'ArrayRef' );

Package main;
use Wibble;
use Moose::Autobox;

my $w = Wibble->new( thingies => [ 'this', 'that' ]);

print join(", ",$w->thingies())."\n";
# prints "this, that"

# here's the bit that I want to make work
$w->thingies->push('another');

print join(", ",$w->thingies())."\n";
# prints "this, that, another"
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And it would do what you want as well.