Each of those SSI calls will be forking off another process and will have no way to communicate up to the parent process that something has happened.
You do have something else that could potentially work. It has been long enough since I have used an SSI that I don't remember if REMOTE_ADDR, HTTP_REFERER, or REMOTE_USER are set inside of an SSI process - I think that they should be (REMOTE_USER would only be set in an htauthed area). As long as even one of those are set, you can use a solution similar to the following. It isn't 100% accurate - but it should be good enough:

use Cache::Memcached;
use Digest::MD5 qw(md5_hex);

sub get_random {
    my ($pick_list, $unique_key) = @_;

    # add uniqueness to our key
    $unique_key .= $ENV{'REMOTE_USER'}
        || $ENV{'REMOTE_ADDR'}  # less reliable
        || $ENV{'HTTP_REFERER'} # least reliable
        || '';                  # based on time now
    $unique_key = md5_hex($unique_key);

    # this solution requires a memcache server
    # or some other cache that handles volitility
    my $mem  = Cache::Memcached->new({servers => ['localhost:11211']})
+;

    # see what has already been used
    my $used  = $mem->get($unique_key) || [];
    $used = [] if @$used >= @$pick_list; # reset when full
    my %used  = map {$_ => 1} @$used;
    my @avail = grep {! $used{$_}} 0 .. $#$pick_list;

    # pick random item and add it to list of used items
    my $index = $avail[rand @avail];
    push @$used, $index;
    $mem->set($unique_key, $used);

    return $pick_list->[$index];
}

# use it like this
my @items = ("http://foo", "http://bar", "http://baz");
my $page  = 'pagefoo';
print get_random(\@items, $page), "\n";
print get_random(\@items, $page), "\n";
print get_random(\@items, $page), "\n";

# it will automatically reset
print "Reset\n";
print get_random(\@items, $page), "\n";
print get_random(\@items, $page), "\n";
print get_random(\@items, $page), "\n";

__END__
Prints something like:
http://bar
http://baz
http://foo
Reset
http://baz
http://foo
http://bar
[download]

You should note that I have used memcached here. My reasoning for doing it is you can have a small localized chunk of memory allocated for this very temporary, very dynamic system and you can insert entries into memcache and then forget about them. The old entries and unused entries are automatically dropped as new entries use up the available memcache space. I would say that for the use you have mentioned here, having a memcache daemon running with only 1MB of allocation would be sufficient for what you are doing.

Oh - and this solution should add very little overhead to your process.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.