3.2.121 ref

ref 

EXPR

The ref operator returns a true value if EXPR is a reference, the null string otherwise. The value returned depends on the type of thing the reference is a reference to. Built-in types include:

REF
SCALAR
ARRAY
HASH
CODE
GLOB

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

See Chapter 4 for more details.