So I was thinking about this some more while waiting for my wife to pick me up. I'm not crazy about making system calls, but it's not a critical feature and I could error-check for the existence of the system commands and if it's not present, I can just make an arbitrary decision. But here was my idea:

sleep 10 | perl -e '$ppid=getppid();$sibs=`pgrep -P $ppid`;print("MY P
+ID: $$\nPARENT: $ppid\nSIBLINGS: $sibs\n")'
MY PID: 79496
PARENT: 563
SIBLINGS: 79495
[download]

It doesn't work all the time. If I run a short command, like echo, I don't get that PID:

echo test | perl -e '$ppid=getppid();$kids=`pgrep -P $ppid`;print("MY 
+PID: $$\nPARENT: $ppid\nSIBLINGS: $kids\n")'
MY PID: 79493
PARENT: 563
SIBLINGS:
[download]

Plus, this doesn't tell me about whether or not I'm running inside a shell script. I couldn't find a perl module that does what pgrep does.