package Bar; use base qw(Foo); # oops. $self->{done} not updated sub naughty_remove { shift->{todo}-- }; ... package Ni; use base qw(Foo); sub naughty_transfer { my ($to, $from) = @_; while ($from->todo) { $to->{todo}++; # oops. $from->{done} not updated $from->{todo}--; }; };