Personally I try to avoid the overloaded DateTime math operators, and only use the comparisons. I'd suggest something like:

my $now = DateTime->now( time_zone => 'Pacific/Honolulu' );
my $one_week_from_now = $now->clone->add( weeks=>1 );
if ( $reservation_date > $one_week_from_now ) ...
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Update: Corrected ->add( week=>1 ) to ->add( weeks=>1 )!

Update 2: Added ->clone, as suggested by tobyink below. That's what I get for running a different test case (DateTime->now->add(weeks=>1)) than what I posted here :-/