Creates the directory specified by FILENAME, with permissions specified by MASK (as modified by umask). If it succeeds it returns true, otherwise it returns false and sets $! (errno). If omitted, MASK defaults to 0777.
In general, it is better to create directories with permissive MASK, and let the user modify that with their umask, than it is to supply a restrictive MASK and give the user no way to be more permissive. The exceptions to this rule are when the file or directory should be kept private (mail files, for instance). The perlfunc(1) entry on umask discusses the choice of MASK in more detail.