According to the documentation,

               Creates the directory specified by FILENAME, with permi
+ssions
               specified by MASK (as modified by "umask").  If it succ
+eeds it
               returns true, otherwise it returns false and sets $! (e
+rrno).
               If omitted, MASK defaults to 0777.

               In general, it is better to create directories with per
+missive
               MASK, and let the user modify that with their "umask", 
+than it
               is to supply a restrictive MASK and give the user no wa
+y to be
               more permissive.  The exceptions to this rule are when 
+the file
               or directory should be kept private (mail files, for in
+stance).
               The perlfunc(1) entry on "umask" discusses the choice o
+f MASK
               in more detail.
[download]

That's all well and good, but on my system, my default umask is 022, so your code still creates a directory as drwxr-xr-x for me. The follwoing seems to do what you want:

perl -e "umask 000; mkdir 'a', 0777";ls -ld a
[download]

I think the docs could be a it more clear that even given a mask for creating directories, mkdir still honors the users umask. - OR I could try READING what I just posted ;)

$,=42;for(34,0,-3,9,-11,11,-17,7,-5){$*.=pack'c'=>$,+=$_}for(reverse s
+plit//=>$*
){$%++?$ %%2?push@C,$_,$":push@c,$_,$":(push@C,$_,$")&&push@c,$"}$C[$#
+C]=$/;($#C
>$#c)?($ c=\@C)&&($ C=\@c):($ c=\@c)&&($C=\@C);$%=$|;for(@$c){print$_^
+$$C[$%++]}
[download]