Untested:

my %seen1;
my @out1 = grep { ! $seen1{ $_->{ID} }++ } @in;
my $arrayref = \@out1;
pp $arrayref;
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Update: Perhaps a bit late for this afterthought, but the general form of this question is "How do I create a reference to X?" Please see perlref and perlreftut. See also Perl Data Structures Cookbook (perldsc) for dealing with complicated referential structures.