I *believe* I have created a much simpler method of reproducing the issue without all of my software necessary. It appears as though on AWS Windows 2012 servers, a ENTER keystroke is required on the server part of the setup before the cpanm gets executed. This is not required on my local win2k8R2 servers.

If it runs through without having to hit ENTER on the server.pl cmd window, that's what I see on my local boxes.

To repro:

- copy below code to client.pl and server.pl respectively
- start the server in the background: 
-- perl server.pl bg
- run the client (different cmd window):
-- perl client.pl
[download]

On my local servers, the server returns to the client properly without any interaction. On my AWS systems, it hangs exactly like my original problem. Hitting ENTER in the server cmd window causes things to go through.

NOTE: there's no method in this test code to stop the server.pl background proc. It has to be killed via Task Manager.

client.pl

use warnings;
use strict;

use IO::Socket::INET;
use Storable;

my $mod = 'IO::Socket::INET';

my $sock = new IO::Socket::INET (
    PeerHost => 'localhost',
    PeerPort => 7800,
    Proto => 'tcp',
);
die "can't create socket\n" unless $sock;

$sock->send("cpanm $mod");

my $recv = Storable::fd_retrieve($sock);

print $$recv;
[download]

server.pl

use warnings;
use strict;

use IO::Socket::INET;
use Proc::Background;
use Storable;

if (@ARGV && $ARGV[0] eq 'bg'){
    Proc::Background->new('perl', $0, 'run');
}
if (@ARGV && $ARGV[0] eq 'run'){
    my $sock = new IO::Socket::INET (
        LocalHost => '0.0.0.0',
        LocalPort => 7800,
        Proto => 'tcp',
        Listen => 5,
        Reuse => 1,
    );
    die "cannot create socket $!\n" unless $sock;

    while (1){
        my $conn = $sock->accept;

        my $cmd;
        $conn->recv($cmd, 1024);

        print "executing: $cmd\n";
        my $ret = `$cmd`;
        print "return: $ret\n";
        Storable::nstore_fd(\$ret, $conn);

        shutdown($conn, 1);
    }

    $sock->close;
}
[download]

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