That's a very nasty side effect and I've experienced that "feature" in a code. And it's also not easy to detect. Imagine you have an X class and you can access it's methods via $o = X->new; But since X does not use strict, you can also access them as static methods like X->foo(123) and if foo() is like this:

sub foo {
   my $self = shift;
   $self->{bar} = shift;
   $self->{bar}
}
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it will succeed. And if you're inheriting several levels from that X class..? I think that every code that does not use strict must be forced to use it. Especially if the code is a CPAN module. There are several modules that do not use strict I think...