On the sender's side we can go ahead and use the Z pack format, since we don't have to try processing it prematurely:

my $str_len = length($data);
$str_len += (4 - $str_len % 4 || 4);
print $sock pack("NZ$str_len", length($data), $data);
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I started out thinking this would be a lot cleaner, but as you can see it doesn't look any better. *shrug*