use Data::Dumper;

my $parent = {
   div => [
      { class => 'A', content => '1' },
      { class => 'A', content => '2' },
      { class => 'A', content => '3' },
      { class => 'B', content => '4' },
      { class => 'B', content => '5' },
      { class => 'B', content => '6' },
   ],
};

my $grouped = {};
for ( @{ $parent->{div} } ) {
   my $class = delete $_->{class};
   push @{ $grouped->{$class} }, $_;
}
$parent->{div} = $grouped;

print(Dumper($parent));