if (open CHILD, "-|") {
  while (<CHILD>) {
    if (/^SUPERSECRETSTRING\n/) {
      exit 0;
    }
    print;
  }
  close CHILD; # to update $?
  exit $? >> 8; # oops, child exits, so we do too
}
... rest of tests here ...
print "SUPERSECRETSTRING\n"; # probably as an END block or DESTROY method