use strict;
use Data::Dumper;

my $data = { 1 => [ 1..10 ],
             2 => [ 11..20 ],
             3 => [ 21..30 ],
};

my $MSB       = 1<<31;
print "$MSB is MSB in decimal\n";

my $SET_MSB   = pack "I",$MSB;
printf "%s SET MASK in binary\n", (unpack "b32", $SET_MSB);

my $UNSET_MSB = pack "I",$MSB-1;
printf "%s UNSET MASK in binary\n", (unpack "b32", $UNSET_MSB);

my $res = serialise($data);
my $dat = unserialise($res);
print Data::Dumper::Dumper($dat);

sub serialise {
    my $data = shift;
    my $str = '';
    for my $doc_id( keys %$data ) {
        $str .= (pack "I", $doc_id) | $SET_MSB;
        for my $pos ( @{$data->{$doc_id}} ) {
            $str .= (pack "I", $pos) & $UNSET_MSB;
        }
    }
  return $str;
}
sub unserialise {
    my $data = shift;
    my $dat = {};
    my $doc_id = undef;
    for( my $i=0;$i<length $data; $i+=4) {
        my $int = unpack "I", (substr $data,$i);
        if ( $int > $MSB ) {
            $doc_id = unpack "I", ((pack "I", $int) & $UNSET_MSB);
        }
        else {
            push @{$dat->{$doc_id}}, $int;
        }
    }
  return $dat;
}