{
    my @nybbles;
    sub c_out {
        push @nybbles, 0   if  ! @_;
        @_= ( @nybbles, map { $_&0xf }
            map { $_, $_>>4, $_>>8 } @_ );
        print OUT pack "C", shift|(shift()<<4)   while  1 < @_;
        @nybbles= @_;
    }
}

{
    my @n;
    sub c_in {
        do {
            return shift(@n) | shift(@n)<<4 | shift(@n)<<8
                if  2 < @n;
            push @n, map { $_&0xf, $_>>4 } unpack "C*", <IN>;
        } while(  1 < @n  );
        return;
    }
}