You're looking for MooseX::AttributeHelpers:

package Wibble;
use Moose;
use MooseX::AttributeHelpers;

has 'thingies' => (
  metaclass => 'Collection::Array',
  is        => 'ro',
  isa       => 'ArrayRef[Str]',
  default   => sub { [] },
  provides  => {
      push     => 'add_thingies',
      elements => 'list_thingies',
  },
);
[download]

This adds wrapper methods into your class to manipulate the underlying arrayref. I've given them sensible names too: add_thingies to push new elements to the end of the array, and list_thingies to return the list of values.

Now you can add additional items like so:

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

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

# here's the bit that we just made work
$w->add_thingies('another');

print join(", ",$w->list_thingies())."\n";
# prints "this, that, another"
[download]

The specific methods you can provide for ArrayRef attributes are listed in MooseX::AttributeHelpers::MethodProvider::List and MooseX::AttributeHelpers::MethodProvider::Array.

I can also recommend the Moose::Manual. It's a nice overview of how Moose works, and describes some of the commonly used extensions.

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