I often write code like this:
$_->store() for @objects;
and quite often it actually works. But suddenly a piece of code doing this in a loop broke with this error message: Can't call method "store" without a package or object reference. And quite right, sometimes @objects would contain plain integers.
Unfortunately it wouldn't be quite easy to track down the relevant change, so enter the perl debugger. Declaring a watch on '@objects' isn't useful as it triggers each time @objects enters or leaves scope. But saving a reference to @objects in $my::objects and the watching $my::objects-> worked.
I had reimplemented the store() method using Data::Walk for walking some structure instead of doing it by hand. And Data::Walk sets $_ to the current node. A due to the aliasing implied in the for-modifier each element in my @objects array was garbled.
Two solutions: The store() method can localize $_ by adding local $_; before using Data::Walk - this works in legacy perl interpreters. The routine looping through @objects can make $_ a lexical variable by adding my $_; before the loop - This is a new feature in Perl 5.10.
An even more robust solution would be to have Data::Walk localize $_ itself AND use a lexical $_ in code where $_ is aliased to important data. See RT #47309.