No, this is a misunderstanding: The rule says "you must", because it doesn't want to leave any doubts to somebody who doesn't know enough to break it with sense. I know that if i want to mess around in some symbol tables (e.g. Export or an Accessor Method Creator...) I will turn off strict 'refs' as well, and I want anybody else to do that then. But someone who isn't sure, must not break the rule.
Same with CGI.pm. There's a "must", because you should not question CGI.pm until you know better.
Tabs: This is not about disk space, but it's no use disuccing this. Tabs aren't evil. :)
To clarify: whom are you making these rules for? Do you need to tell yourself "use strict, you(=I) know when I may disable it!"? Such guidelines are usefull in two ways: a) for people who learn Perl, for them I wrote the "MUST". To say "SHOULD" tells them there are exceptions, but there are no exceptions they need to know about yet. b) For a project, to keep it all consistent and easier to make many people work on the same Code. There the "MUST" fits because an exception should be rare and known, because it is an exception. And: I wouldn't work with people who need to be told to use "strict" and when to disable it.