You may find a wrapper like this helpful:

sub h {
  tie my %hash, 'Tie::Hash::MultiValueOrdered';
  while (my ($k, $v) = splice @_, 0, 2) {
    $hash{$k} = $v;
  }
  \%hash;
}
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That way you can build your data structure pretty naturally, just using h(...) for a hashref instead of {...}:

my $people = [
  h( name => "Alice", age => 21 ),
  h( name => "Bob", name => "Robert", age => 22 ),
];

print JSON::MultiValueOrdered->new(pretty=>1)->encode($people);
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