Very nice...thank you it is what I needed.
This is running on HPUX and the one I found that works is the following:

$tpgrp = tcgetpgrp(fileno(*TTY));
$pgrp = getpgrp();
if ($tpgrp == $pgrp) {
    system("which which > whichtest");
} else {
    print "Skipping which as we are not interactive.\n";
}
[download]

I needed a change in this as the system call on HPUX gets the shell into a waiting state until it receives some input, which does not work well if we run it as a background process. Rather than skip doing this as a check, which we could do as the script is run manually at times, I wanted a way to have it run only when we needed it.