To solve your larger problem of calling arbitrary builtins with the appropriate parameters, I think you will have to explicitly pass the parameters:

mkdir $a[0], $a[1];
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That won't work, it'll just create the directory $a[0] with the permissions as per filemask $a[1] (at least that's what it does on my 5.8.8 on Linux, is this different on Windows?)

To the OP: mkdir expects a single directory name. If you want to create multiple directories just do so in a loop, e.g.

for my $d (@a) {
  mkdir $d or die "Couldn't create directory $d: $!";
}
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