In the headline, you are asking a Moose question but in the code you are using Moo. Does Moo support 'ArrayRef' etc as well? If I change use Moo; to use Moose; and follow moritz' advise, I get no error anymore. I am not too familiar with Moo and Moose, just a guess..

UPDATE: Just found this on http://search.cpan.org/~mstrout/Moo-1.001000/lib/Moo.pm:

isa

Takes a coderef which is meant to validate the attribute. Unlike Moose, Moo does not include a basic type system, so instead of doing isa => 'Num', one should do

 isa => sub {
   die "$_[0] is not a number!" unless looks_like_number $_[0]
 },
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