use warnings;
use strict;

{ package Foo;
	use Data::Dump;
	use Scalar::Util qw/blessed/;
	sub new { return bless {}, shift }
	sub foo {
		my $self = shift if blessed($_[0]) && $_[0]->isa(__PACKAGE__);
		dd 'foo', $self, \@_;
	}
	sub bar {
		my $self = shift if defined blessed($_[0])
			&& $_[0]->isa(__PACKAGE__);
		dd 'bar', $self, \@_;
	}
}

{ no strict 'refs'; @{"0::ISA"} = ('Foo') }

my $x = bless {}, '0';
$x->foo();  # ("foo", undef, ["0=HASH(0x561239194f3e)"])
$x->bar();  # ("bar", "0=HASH(0x561239194f3e)", [])