Use blessed in Scalar::Util instead of testing for ref:

use strict;
use warnings;
use Scalar::Util 'blessed';

wants_foo_object({});

sub wants_foo_object {
    die "no object" unless defined(my $obj = shift);
    die "not an object" unless blessed($obj);
    die "not a foo" unless $obj->isa('foo');
}
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