On the first morning of the 10th German Perl Workshop Max did a tutorial on Advanced Perl during which someone asked about the difference between
use vars. Max didn't know the answer offhand (and being smart, doesn't need to pretend to be by pretending to know the answer), and I had to scratch my head a bit when Jürgen told me about it (I'd been in the other tutorial), so Jürgen asked if I could do a lightning talk to explain it. I said yes (which seemed to be consistent with a question at breakfast - Jonathan asked "are you speaking this year?" and I replied "not yet").
However, today it turned out that it is a good thing that I am a virtuously lazy programmer and hadn't written any slides, as Steffen Ullrich did an excellent talk on Perl Gotchas which had an example of code which explained it much better than I could. Specifically, consider the following:
our @ISA = 'awk';
our @ISA = 'urkk';
@ISA = 'rakkk';
$ perl -Mawk -e0
but nevertheless buggy. Why? Because the inheritance isn't set up correctly:
$ perl -Mawk -wle 'print @owww::ISA'
$ perl -Mawk -le 'print @rakkk::ISA'
Erk! How did that happen? It's because the our was missed from the last assignment. It should read:
our @ISA = 'rakkk';
But without that our, that last assignment is still to the variable
@rakkk::ISA , not
our actually creates a lexical alias to the global variable, a lexical that is still in scope for the rest of the file.