But it *is* answering the question asked. 'Is /mnt/blahblah' writeable now' is not the same question as 'is /mnt/blahblah writeable after it has been autommounted'. You are asking 'is it writeable now'. The canonical way of asking that question is a stat(2) call. As I demonstrated, that is what perl does. It's what your shell probably does too - mine certainly does:
$ strace sh -c "if [ -w /etc ]; then echo Writeable; fi"
...
stat("/etc", {st_mode=S_IFDIR|0755, st_size=12288, ...}) = 0
...
Which clearly shows that perl is Doing The Right Thing by using stat(2) to check whether the directory is writeable.