Moose::Manual::Attributes says: "If you want to use a reference of any sort as the default value, you must return it from a subroutine. has 'mapping' => ( is => 'ro', default => sub { {} }, );

So perhaps:

has 'url' => (is => 'rw', isa => 'HashRef', default => sub{ {'url1' =>
+ 'perlmonks.org'}}, );
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I can't test; I've never used Moose.