my $obj= MyClass->new;
my $immuobj= MyClass->new;
[download]

Shouldn't that be

my $obj= MyClass->new;
my $immuobj= MyImmuClass->new;
[download]

Anyway, this doesn't change the result. The difference between $obj->size(27) and $obj->setsize(27) is probably caused by isa => 'Int'. size is checking that argument is int, and setsize does not.