For example, you get a package from another person, and he didn't "use strict" in his package. Now you use his package in your package, and you specify "use strict". If this "use strict" is not per lexical body scope, but affects the whole script(remember his package is inside your script scope now.), Perl will probably refuse to run your script, because of all those problems in his package caused by your "use strict".
Like CGI.
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We are the carpenters and bricklayers of the Information Age.
Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.