Programming Perl, 4th Edition

warnings

use warnings;   # same as importing "all"
no warnings;    # same as unimporting "all"

use warnings::register;
if (warnings::enabled()) {
    warnings::warn("some warning");
}

if (warnings::enabled("void")) {
    warnings::warn("void", "some warning");
}

warnings::warnif("Warnings are on");
warnings::warnif("number", "Something is wrong with a number");

This lexically scoped pragma permits flexible control over Perl’s built-in warnings, both those emitted by the compiler as well as those from the runtime system.

Once upon a time, the only control you had in Perl over the treatment of warnings in your program was through either the –w command-line option or the $^W variable. Although useful, these tend to be all-or-nothing affairs. The –w option ends up enabling warnings in pieces of module code that you may not have written, which is occasionally problematic for you and embarrassing for the original author. Using $^W to either disable or enable blocks of code can be less than optimal because it works only during execution time, not during compile time.^[263] Another issue is that this program-wide global variable is scoped dynamically, not lexically. That means that if you enable it in a block and then from there call other code, you again risk enabling warnings in code not developed with such exacting standards in mind.

The warnings pragma circumvents these limitations by being a lexically scoped, compile-time mechanism that permits finer control over where warnings can or can’t be triggered. ...