If say, you had 50 ips in the array, and a block of 40 of them where duds, then you wouldn't get a very load balanced selection from the remaining 10. The burden would largely keep falling on a single ip and you'd probably be just as well off looping through the array sans random?

Instead of looping around the array, I would remove each try from the remaining pool:

use IO::Socket;
my @ips = ('XXX.XXX.XXX.X0','XXX.XXX.XXX.X1','XXX.XXX.XXX.X2');
my $port = XXXX;

my $sock;
while (@ips) {
  # radmomly pick an ip from the list (removing it at the same time)
  my $ip = splice(@ips, rand(@ips), 1);

  # try to connect
  $sock = IO::Socket::INET->new(  PeerAddr => $ip.':'.$port,
                                  Blocking => 1,
                                  Timeout  => 1,
                               );
  # success!
  last if ($sock);
}
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I did like your negative index trick!