The use should be as close to the first thing executed as possible, otherwise you may think you have strict turned on, but won't.
Use of this in modules isn't encouraged as you don't really gain anything other than a shaving off a few keystrokes. Modules are generally not invoked as if executables from the command line. Additionally there would be a slight (not timed/tested) overhead of doing this a large amount of times. Therefore, stick to use in non-modules.
Not tested under mod_perl. While I don't suspect any problems, mod_perl does weird things.
This doesn't enabled strict, warnings, etc for all modules/other perl associated with that script -- only to the end of the file.