use constant EX_TEMPLATE => 'II';
my $length = 8;
my %ValArrayByID = ();
# ... open file handle $fh, etc...
my $packed_data;
while (read($fh, $packed_data, $length)) {
my ($ID, $Val) = unpack EX_TEMPLATE, $packed_data;
not defined $ValArrayByID{$ID}
    and $ValArrayByID{$ID} = [];
push @{$ValArrayByID{$ID}}, $Val;
}
# ... close file and use my newly minted hash