... I'm a little less sure about the first unpack to split the $data scalar into the list of items.

Can you say more about why you are unsure about this statement? Is it because of the interpolated  $entry_size component in the unpack template? If so, be assured that this is a perfectly kosher maneuver: a template specification string is just a string no matter how compounded. (Sometimes a string is just a string...)

Update 1: Changed a word, added a link.

Update 2: Hey, this is very late, but I just noticed that the unpack documentation includes an example of this exact technique in a limited implementation of substr:

sub substr {
    my($what,$where,$howmuch) = @_;
    unpack("x$where a$howmuch", $what);
}
[download]