# untested

use Sort::Key::Radix qw(ikeysort);

my $packed_data = do { local $/; <$fh> };
my $last = length($packed_data) / 8 - 1;

my @sorted_ix = ikeysort { unpack I => substr($packed_data, $_ * 8, 4) } 0..$last;

# now iterating over @sorted_ix, you get the data sorted by Id:
for my $ix (@sorted_ix) {
    my ($id, $val) = unpack II => substr($packed_data, $_ * 8, 8);
    print "id: $id, val: $val\n"
}