It looks like you have the part that gets the titles from the DB correct. Consider this:

use strict;
use warnings;
use 5.10.0;
use JSON;
use Data::Dump qw(pp);

my %output;

foreach my $title ("Valls", "AUT15605", "10UT15605")
{
   push @{$output{items}}, {title => $title};
}
my $output_json = encode_json \%output;
say $output_json;
pp \%output;

__END__

This is what the Perl structure looks like:
{
  items => [
    { title => "Valls" },
    { title => "AUT15605" },
    { title => "10UT15605" },
  ],
}

This is the encoded JSON:

{"items":[{"title":"Valls"},{"title":"AUT15605"},{"title":"10UT15605"}
+]}
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"items" is a hash key which has a value that is a reference to an array of anon hashes.