#!/usr/bin/perl
  
  use Your::Module;
  use Proc::Background;
  use Test::Resub qw(resub);

  # disable Proc::Background->new for the scope of this block
  {
    my $fake_new = sub {
      return bless {}, 'Proc::Background'; # or whatever
    };
    my $resub = resub 'Proc::Background::new', $fake_new;

    # continue on your merry way, content knowing that
    # your $fake_new is getting called.
  }
  # scope of $resub object ends, its DESTROYER restores
  # the original Proc::Background::new.

##</code><code>##

  package My::Module;

  use IO::Socket::INET;

  ...

  sub open_socket_to {
    my ($class, %args) = @_;
    return IO::Socket::INET->new(
      PeerAddr => $args{machine},
      PeerPort => $args{port},
      Proto => $args{protocol} || 'tcp',
      Type => SOCK_STREAM,
    );
  }

##</code><code>##

  use Test;
  ...
  use My::Module;
  use Test::Resub qw(resub);
  use IO::Socket::INET;

  # test open_socket_to
  {
    my $rs = resub 'IO::Socket::INET::new', sub { 'a socket' }, capture => 1;
    my $socket = My::Module->open_socket_to(
      machine => 'example.com',
      port => '-29',
    );

    # return value of ->open_socket_to is whatever IO::Socket::INET::new gives us
    ok( $socket, 'a socket' );

    # We created the socket correctly - the fact that we provided an
    # illegal PeerPort doesn't need to be handled by this test:
    # IO::Socket::INET should have his own tests for his ->new.
    is_deeply( $rs->named_method_args, [{
      PeerAddr => 'example.com',
      PeerPort => -29,
      Proto => 'tcp',
      Type => SOCK_STREAM,
    }] );
  }