sub log2{ int( ( log( $_[0] ) + 1e-15 ) / log( 2 ) ) }

sub PP_EliasPack {
    my $packed = '';
    my $out = 0;
    for my $num ( @_ ) {
        my $len = log2( $num );
        $out += $len;
        vec( $packed, $out++, 1 ) = 1;
        vec( $packed, $out++, 1 ) = ( $num & ( 1 << $_ ) ) ? 1 : 0
            for 0 .. $len - 1;
    }
    return $packed;
}

sub PP_EliasUnpack {
    my $packed = shift;
    my $bits = length( $packed ) * 8;
    my $in = 0;
    my @unpacked;
    while( $in < $bits ) {
        my( $len, $num ) = ( 0 ) x 2;
        $len++ while $in < $bits && vec( $packed, $in++, 1 ) == 0;
        last if $in == $bits;
        vec( $packed, $in++, 1 ) and $num |= ( 1 << $_ ) for 0 .. $len - 1;
        $num |= ( 1 << $len );
        push @unpacked, $num;
    }
    return @unpacked;
}

##</code><code>##

C:\test>678848
Run with 15000 unique words in 1000 documents (Ave: 554 words/doc)

ASCII    uses 4755336 bytes
W-BER    uses 3196819 bytes
Binary   uses 3279128 bytes
Elias    uses 3980063 bytes
PP_Elias uses 3980063 bytes

1 trial of Packing ascii      (10.203s total)
1 trial of Unpacking ascii     (3.159s total)

1 trial of Packing W-BER      (18.159s total)
1 trial of Unpacking W-BER     (1.516s total)

1 trial of Packing binary      (9.910s total)
1 trial of Unpacking binary    (1.455s total)

1 trial of Packing Elias      (13.613s total)
1 trial of Unpacking Elias     (2.739s total)

1 trial of Packing PP_Elias   (31.128s total)
1 trial of Unpacking PP_Elias (18.094s total)