#file: test.Mongo.pl
 use strict;
 use warnings;

 use MongoDB; # also loads MongoDB::MongoClient
 use Data::Dumper;

 my $connect_string=$ARGV[0];
 my $client = MongoDB::MongoClient->new({'host' => $connect_string});
 my $doc    = $client->get_database('mydb')->get_collection('mycoll')->find_one();
 $client->disconnect;
 print Dumper $doc;

##</code><code>##

#file: test.Mango.pl

 use strict;
 use warnings;

 use Mango;
 use feature 'state';
 use Data::Dumper;

 my $connect_string=$ARGV[0];
 sub mango { state $m = Mango->new($connect_string)}
 my $doc = mango->db('mydb')->collection('mycoll')->find_one();
 print Dumper $doc;

##</code><code>##

>  perl test.Mango.pl "$MONGO_STRING"       #OK.5
-- Emit connect in Mojo::IOLoop::Client (1)
-- Emit write in Mojo::IOLoop::Stream (0)
-- Emit drain in Mojo::IOLoop::Stream (0)
-- Emit read in Mojo::IOLoop::Stream (1)
-- Emit write in Mojo::IOLoop::Stream (0)
-- Emit drain in Mojo::IOLoop::Stream (0)
-- Emit read in Mojo::IOLoop::Stream (1)
-- Emit write in Mojo::IOLoop::Stream (0)
-- Emit drain in Mojo::IOLoop::Stream (0)
-- Emit read in Mojo::IOLoop::Stream (1)
-- Emit write in Mojo::IOLoop::Stream (0)
-- Emit drain in Mojo::IOLoop::Stream (0)
-- Emit read in Mojo::IOLoop::Stream (1)
-- Emit connection in Mango (0)
-- Emit finish in Mojo::IOLoop::Delay (1)
-- Emit write in Mojo::IOLoop::Stream (0)
-- Emit drain in Mojo::IOLoop::Stream (0)
-- Emit read in Mojo::IOLoop::Stream (1)
$VAR1 = {
          '_id' => bless( {
                            'oid' => '_L+.\''
                          }, 'Mango::BSON::ObjectID' ),
            .....          
        };

##</code><code>##

>  perl test.Mango.pl "$ATLAS_STRING"       # ERROR:  Premature connection close at /var/..../local/lib/perl5/Mango/Cursor/Query.pm line 112. 
-- Emit connect in Mojo::IOLoop::Client (1)
-- Emit write in Mojo::IOLoop::Stream (0)
-- Emit drain in Mojo::IOLoop::Stream (0)
-- Emit close in Mojo::IOLoop::Stream (2)
Premature connection close at  /var/..../local/lib/perl5/Mango/Cursor/Query.pm line 112. 

##</code><code>##

Mojo::IOLoop::Stream::_read(/var/..../local/lib/perl5/Mojo/IOLoop/Stream.pm:103):
103:      return if $! == EAGAIN || $! == EINTR || $! == EWOULDBLOCK;
  DB<8> x $!
0  'Connection reset by peer'

##</code><code>##

my %extra;
$extra{replica_set_name} = 'xxxx';
$extra{tls} = 1;        #Note, it's "tls" (not "ssl" like it would be in the connection string)
sub mango { state $m = Mango->new($connect_string, %extra);}