No, this is not caused by numerical equivalence to zero. Nor is it caused by Infinity/inf differences:

@a = (5 .. 10);
print
$a['inf'], "\n",
$a[-inf], "\n",
-inf==0 ? "-inf is zero\n" : "-inf isn't zero\n",
'inf'==0 ? "inf is zero\n" : "inf isn't zero\n";

# still outputs 10 and 5 and states that the infinities aren't zeroes
[download]

perl --version says "This is perl, v5.8.8 built for x86_64-linux-thread-multi"

Update: The weird thing is that indexing by -inf gives the first element and indexing by inf gives the last one. That's not what I'd expect.