Here's one way to tell, extracted from an example in the Perl Developer's Dictionary:

# To be honest: if you're not sure what's in $t
#   at all, then things begin to get silly.
if ($type = ref $t) {
        eval { $t->isa("UNIVERSAL"); };
        if ( $@=~/unblessed/ ) {
                print "It's a $type, not blessed.\n"
        } else {
                print "It's blessed into class $type";
        }
} else {
        print "Not a reference at all\n";
}
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