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

If the referenced object has been blessed into a package, then that package name is returned instead. 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//.

See also perlref.