I believe there's an environment variable that should hold that information (or, at least, the path to the file that's executing the SSI). Do a dump of %ENV in your SSI to check:

print "Content-Type: text/html\n\n";
for my $k (keys %ENV) {
    print $k, " = ", $ENV{$k}, "\n";
}
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I know that such a variable exists; I just don't remember its name (I also remember that there is/was a bug with Netscape Enterprise where the variable doesn't get set-- it worked on Apache, though.)