Just to throw a wrench in the works ;-) The binary data is packed in Network bit order, won't pulling it out using C2 not respect that? So i tried this:

my ($data) = unpack "N", $foo;
my ($ver, $hlen) = (vec($data,0,4), vec($data,1,4));
my $tos = vec($data, 1, 8);
my $len = vec($data, 1, 16);
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Which returned completely un-expected results. Is it not possible to use vec on data pulled using "N"? (yes, i know it can be more compact, split up for readability.)