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

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";
}
if (UNIVERSAL::isa($r, "HASH")) {  # for subclassing
    print "r is a reference to something that isa hash.\n";
}

See also perlref.