Using HTML::TokeParser you can grab the SSI and then split it on the --s to grab the variable:

use diagnostics;
use strict;
use warnings;

use HTML::TokeParser;

my $p = HTML::TokeParser->new("ssi.html") || die $!;

my $randomVar;

while (my $token = $p->get_token) {
    my $tokenType = shift @{$token};

    if ($tokenType eq "C") {
        # add another loop in to check if it's the right ssi
        my @parts = split /--/, shift(@{$token});
        $randomVar = $parts[2];
    }
}

print $randomVar, "\n";
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Hope that helps.