Or:

my (@cmds) = ("HELO", "dom:$domain", "ipaddr:$ipaddress", "addzone");

while (@cmds)
{
    print $ns1 shift(@cmds), "\n";
    $line = <$ns1>;          # don't need the sleep;
                             # the read should block until
                             # a line is available.

    defined($line) or last;  # quit if connection closed
                             # (could die instead)

    print $line;             # don't need \n now, $line
                             # not chomped

    ($line =~ /^CAIO$/) and last;
                             # $ can handle trailing
                             # newline, with no
                             # significant speed penalty
                             # (should be optimized by
                             #  regex engine)
}
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