use strict;
use warnings;

wants_foo_object({});

sub wants_foo_object {
	die "no object" unless defined(my $obj = shift);
	die "not an object" unless ref($obj);
	die "not a foo" unless $obj->isa('foo'); #the error I want
}

#The error I get: 
#C:\thomasdata\experiments>perl unblessedWantBetterError.t
#Can't call method "isa" on unblessed reference at unblessedWantBetterError.t lin
#e 10.