Other Monks have given you the answer to your question and advice on how to access the documentation. This is to bring to your attention, should you be working in a *nix environment, the fact that a symbolic link to a directory will also give a positive with the -d file test operator. If you wish to distinguish between directories and symbolic links that point to directories you will also have to use the -l file test.

knoppix@Microknoppix:~/data$ mkdir mydir
knoppix@Microknoppix:~/data$ ln -s mydir mylink
knoppix@Microknoppix:~/data$ ls -l
total 4
drwxr-xr-x 2 knoppix knoppix 4096 Aug 13 15:52 mydir
lrwxrwxrwx 1 knoppix knoppix    5 Aug 13 15:52 mylink -> mydir
knoppix@Microknoppix:~/data$ perl -E '
> say q{-} x 40;
> foreach my $thing ( qw{ mydir mylink } )
> {
>     say
>        qq{-d test says $thing is },
>        -d $thing
>        ? q{}
>        : q{not },
>        q{a directory};
>     say
>        qq{-l test says $thing is },
>        -l $thing
>        ? q{}
>        : q{not },
>        q{a symlink};
>     say q{-} x 40;
> }'
----------------------------------------
-d test says mydir is a directory
-l test says mydir is not a symlink
----------------------------------------
-d test says mylink is a directory
-l test says mylink is a symlink
----------------------------------------
knoppix@Microknoppix:~/data$
[download]

I hope this is helpful.