perl -MData::Dumper -E 'my $r=[map { id=>$_ }, ("b","a".."c","b")]; 
     say Dumper $r;
     my %h;
      $h{$r->[$_]{id}}++ and delete $r->[$_] for  0..$#$r;
    say Dumper $r'
--- SECOND (Relevant) PART of OUTPUT---
$VAR1 = [
          {
            'id' => 'b'
          },
          {
            'id' => 'a'
          },
          undef,
          {
            'id' => 'c'
          }
        ];