use threads       qw( async );
use Thread::Queue qw( );

my $q = Threads::Queue->new();

async { 
   while (<STDIN>) {
      $q->enqueue(...);
   }
   $q->enqueue(...);
};

async { 
   while (<$fr_child_pipe>) {
      $q->enqueue(...);
   }
   $q->enqueue(...);
};

while (defined(my $item = $q->dequeue())) {
   ...
}