Why doesn't unpack "P", 0 dump? I expected it to and this confused me.

The reason (on some level) is that pp_unpack() calls newSVpvn(), which calls sv_setpvn(), which has this code:

    if (!ptr) {
        (void)SvOK_off(sv);
        return;
    }
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In other words, unpack("P", 0) just returns an undefined SV. Since C code often uses null pointers in the same way that Perl code uses undef, I think this is more useful than crashing. :-)