I tried the two samples.
The second one is the one I like it (because conceptually you don't have to create a type not related to class in the base-class).

Just a little precisation:
Here:

$args{pages} = [ map Page->new($_), @$pages ];
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must be:

$args{pages} = [ map Page->new(URL=>$$_{Page}{URL})), @$pages ];
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And I have a question:

I have also checked Moose manual:

What "around" is?

around BUILDARGS => sub {
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Thank you for your answers.