Perl Security: 3. Reporting warnings
Granularly controlled warnings

The warnings are divided into categories, and each of them can be turned on or off:

use warnings qw(once unopened);
no warnings qw(redefine);

will issue warnings when a symbol is used only once or when an unopened filehandler is used. Warnings about functions being redefined will not be issued.

Warnings can be labeled as fatal, terminating program execution:

use warnings FATAL => qw(void);

will terminate the program whenever a void context warning is issued.