Indeed, sh is linked to dash:

$ ls -al `which sh`
lrwxrwxrwx 1 root root 4 Mar  1  2012 /bin/sh -> dash
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Actually, I don't need all these forked debuggers opening up since I'm not debugging the (trivial) forked processes. (UPDATE: I discovered that  DB<1> undef $DB::inhibit_exit allows me to do this.)

I suppose I could go into the source and change it to use bash instead of dash (in the guise of sh) and see if that at least forks working debuggers.