sub class_or_instance
{
    my $invocant = shift;

    if (ref($invocant))
    {
        # got an instance (i.e., a "self" reference)
    }
    else
    {
        # got a class name (hopefully anyway ... 8-)
    }
}

...

Foobar->class_or_instance()  # passes class name "Foobar" in as @_[0]
my $f = Foobar->new();
$f->class_or_instance();     # passes object $f in as @_[0]