Remember: it's the referant, not the reference that's blessed - so taking a reference to $obj_var_a can give you an object that overloads ==. Consider:

my $obj_var_a = 42;
my $o = bless \$obj_var_a, 'OverloadedClass';
my $ref = \$obj_var_a;
print "overloaded" if $ref->isa('OverloadedClass');
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Don't you just love exceptions like this :-)