use warnings pragma rather than -w on shebang line

[vitahall]

This allows for simple, portable use of /usr/bin/env in the shebang line, because then you don't need to split perl -w on any platform.

There shouldn't be any change in behavior from perl -w, and use warnings is compatible back to Perl 5.6 from 2000.

[daveewart]

Looks fine, thanks, will merge.