Returns a non-empty string if EXPR is a reference, the empty
string otherwise.  If EXPR is not specified, $_
 will be used.  The
value returned depends on the type of thing the reference is a reference to.
Builtin types include:
- SCALAR
- ARRAY
- HASH
- CODE
- REF
- GLOB
- LVALUE
- FORMAT
- IO
- VSTRING
- Regexp
You can think of ref as a typeof
 operator.
The return value LVALUE
 indicates a reference to an lvalue that is not
a variable.  You get this from taking the reference of function calls like
pos() or substr().  VSTRING
 is returned if the reference points
to a version string.
The result Regexp
 indicates that the argument is a regular expression
resulting from qr//.
If the referenced object has been blessed into a package, then that package
name is returned instead.  But don't use that, as it's now considered
"bad practice".  For one reason, an object could be using a class called
Regexp
 or IO
, or even HASH
.  Also, ref doesn't take into account
subclasses, like isa
 does.
Instead, use blessed
 (in the Scalar::Util module) for boolean
checks, isa
 for specific class checks and reftype
 (also from
Scalar::Util) for type checks.  (See perlobj for details and a
blessed/isa
 example.)
See also perlref.