That's fine, but you didn't provide a sample that repros on your coworker's machine, nor say what type of machine it is. Investigation cannot happen if you do not help others help you. Even if perl defined the behavior clearly, a bug may exist (even in your code).
The problem is that he used *my* code, and when we tried to
reproduce the problem, it worked well all the time. That's why
I was wondering whether this is the type of undefined behaviour which "just works well most of the time". The machines are of the same type, but he uses Perl 5.8.8, while I am using 5.8.7
and 5.10 (ActiveState always). For this reason, I was interested whether the Perl programming langauge itself defines the behaviour (i.e. whether Perl is *supposed* to close the file when the local handle goes out of scope), in which case we should persuade the problem further,
or whether the behaviour is unspecified/implementation dependent, in which case it makes no sense doing further research.