Scalar::Util::blessed is the way to do this. If you're exceptionally paranoid, you have to check if it's defined, not merely true.

use Scalar::Util 'blessed';
my $oh_bee_jay = bless {}, '0';
print "not blessed\n"      if not     blessed $oh_bee_jay;
print "defined blessed\n"  if defined blessed $oh_bee_jay;
__END__
not blessed
defined blessed
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I'd be surprised if this is an issue, though. Package '0' is not too popular.