use warnings;
use strict;
use JSON::MaybeXS;

my $rows   = [["val1"],["val2"]];
my $rows_a = [[],["val2","val3"]];
my $return;
$return->{'key1'}   = [join(', ', map { $_->[0] } @$rows)];
$return->{'key2'}   = [map { $_->[0] } @$rows];
$return->{'key3'}   = [map { @$_     } @$rows];
$return->{'key2_a'} = [map { $_->[0] } @$rows_a];
$return->{'key3_a'} = [map { @$_     } @$rows_a];
print encode_json($return), "\n";

__END__
{
"key1":["val1, val2"],
"key2":["val1","val2"],
"key3":["val1","val2"],
"key2_a":[null,"val2"],
"key3_a":["val2","val3"]
}