our $conn already declares the name for a global variable.

What is the specific problem you encounter?

my problem is that i don't know clearly how to build and manage mongodb connection pool, i want to make a pool by the code. but i don't think it's right. my thinking is that step 1, a global varaiable "new instance", and step 2, some clients share this instance or get connection from the pool.

The easy way to manually a connection (pool) is to use a function to get a connection:

use MongoDB;
our $mongodb;

sub mongodb() {
  if(!defined($mongodb)){
    $log->info("Creating new DB connection");
    $mongodb =  MongoDB::MongoClient->new;
  } else{
    $log->info("DB connection already exists");
  }
  return $mongodb
};
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And then everywhere in your code, you don't use $mongodb but mongodb() instead.

If you want to extend that to a connection pool, you can either have your code manually return finished connections to the pool or write something that makes this automatic.

my $max_mongodb_connections = 4;
our @mongodb_pool;
our %used_connections;

sub mongodb() {
  my $mongodb;
  if(@mongodb_pool){
    $log->info("Using existing DB connection from pool");
    $mongodb = shift @mongodb_pool;

  } elsif(@mongodb_pool and keys %used_connections < $max_mongodb_conn
+ections){
    $log->info("Creating new DB connection");
    $mongodb =  MongoDB::MongoClient->new;
  } else{
    $log->info("Need to wait until a mongodb connection becomes availa
+ble");
  }

  $used_connections{ $mongodb } = 1 if $mongodb;
  return $mongodb
};

sub return_connection {
  my( $mongodb )= @_;
  delete $used_connections{ $mongodb };
  unshift @mongodb_pool, $mongodb; # keep the hot connections hot
}
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