You asked the question; you don't deserve a lecture.
The answer is you can disable unwanted warnings by
coding, possibly in a block, with the no warnings 'once' syntax.
This disables the 'once' warning. But you did not want to
disable the 'once' warning, you wanted to disable another warning
whose name you do not know! Than is usually the case.
Simple, aim your browser to the
warnings.h file (mine is at /usr/lib/perl/5.8.7/CORE/warnings.h ),
then guess the name of the warning from the definitions.