I'm not sure why you would need this in the constructor, but altering a little the example from kennethk, this is how I would do it:

use Moose;
use JSON;

has content => (
    isa => 'Str',
    is  => 'ro',
    required=> 1,
);

has results => (
    isa => 'Str',
    is  => 'ro',
    lazy => 1,
    builder => '_build_results',
);

sub _build_results {
    my $self = shift;
    return decode_json($self->content);
}
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With lazy and builder, the results attribute will be populated only when needed, only when you call $obj->results()

Note: the builder method has access to $self, the reference to the instance, because it is called AFTER the instance is created, however, in kennethk's example, I don't think the default sub has access to $self or in this case $_[0]* because the default sub is executed WHILE the instance is being created, therefore, the reference is not yet available.

* In a method, $_[0] contains the reference to the instance, same as if you do my $self = shift; or my $self = shift(@_); or my $self = $_[0];. I may be mistaking but I've had problems with this before and since then, I use lazy attributes with a builder method.