ref EXPR
ref

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.

if (ref($r) eq "HASH") {
    print "r is a reference to a hash.\n";
}
unless (ref($r)) {
    print "r is not a reference at all.\n";
}

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.