Opens a directory named EXPR for processing by readdir, telldir,
seekdir, rewinddir, and closedir. Returns true if successful.
DIRHANDLE may be an expression whose value can be used as an indirect
dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
scalar variable (or array or hash element), the variable is assigned a
reference to a new anonymous dirhandle.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.
See example at readdir.