sub packwithsubst {
    my $string = shift;
    my $bits = unpack 'b*', $string;
    $bits =~ s/(.{7})./$1/g;
    return pack 'Cb*', length($string), $bits;
}

sub unpackwithsubst {
    my ($length, $chars) = unpack 'Cb*', shift;
    $chars =~ s/(.{7})/${1}0/g;
    substr $chars, $length * 8, 8, '';
    return pack 'b*', $chars;
}