my $buffer = "";
my $BYTES = 5;
my $BITS_IN_BYTE = 8;
my $FULL_BYTE = (1 << $BITS_IN_BYTE) - 1; 

sub store {
    my ($pos, $value) = @_;
    for (0 .. $BYTES - 1) {
        vec($buffer, $pos * $BYTES + $_, $BITS_IN_BYTE) =
           ($value >> ($BITS_IN_BYTE * ($BYTES - 1 - $_))) & $FULL_BYTE;
    }
}

sub fetch {
    my ($pos) = @_;
    my $sum = 0;
    for (0 .. $BYTES - 1) {
        $sum <<= $BITS_IN_BYTE;
        $sum += vec($buffer, $pos * $BYTES + $_, $BITS_IN_BYTE);
    }
    $sum;
}

#
# Testing
#
my $TEST_SIZE = 1_000_000;
my @offsets = map {int rand 1 << ($BYTES * $BITS_IN_BYTE)} 1 .. $TEST_SIZE;

for (my $i = 0; $i < @offsets; $i++) { 
    store $i, $offsets[$i];
}
for (my $i = 0; $i < @offsets; $i++) { 
    my $val = fetch $i;
    die unless $val == $offsets[$i];
}

use Devel::Size 'size';
use 5.010;

say "Index size: ", size $buffer;
__END__
Index size: 5000044