G'day learning.moose,

default => sub { { 'url1' => 'perlmonks.org'} }
[download]

It's documented in Moose. Look for has and its subsection default.

"... if you wish to initialize it with a HASH or ARRAY ref, then you need to wrap that inside a CODE reference."

If you really are learning Moose then you should read all of that (fairly short) document. It provides basic information with links to more detailed information and advanced topics.