According to perlrun:

For security reasons, this option must be seen by Perl quite early; usually this means
it must appear early on the command line or in the #! line ...

And that makes sense, since the switches are parsed first come first serve. Also, many places require warnings to be "turned off" for production code, as using them does require a few more CPU cycles -- some places, however, turn them off because of plethora of warnings being sent out to their error logs is overwhelming. :P