Creates the directory specified by FILENAME, with permissions specified by MODE (as modified by
umask). If it succeeds it returns true; otherwise it returns false and sets
$! (errno). MODE defaults to 0777 if omitted, and FILENAME defaults to
$_ if omitted.
In general, it is better to create directories with a permissive MODE and let the user modify that with their
umask than it is to supply a restrictive MODE 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 documentation for
umask discusses the choice of MODE in more detail. If bits in MODE other than the permission bits are set, the result may be implementation defined, per POSIX 1003.1-2008.
Note that according to the POSIX 1003.1-1996 the FILENAME may have any number of trailing slashes. Some operating and filesystems do not get this right, so Perl automatically removes all trailing slashes to keep everyone happy.