Not 'use warnings', but -w. The warnings pragma came into the Perl distrobution I believe in 5.005_63, so not everyone will have it, but everyone has -w. On an aside, of you are using a pragma, or something which came out in a version of Perl (for example, say you use our(), which came in 5.6), you may want put at the top of your script:
require 5.6;
Cheers,
KM