Programming Perl, 3rd Edition

use diagnostics

use diagnostics;            # compile-time enable
use diagnostics -verbose;

enable  diagnostics;        # run-time enable
disable diagnostics;        # run-time disable

This pragma expands the normal, terse diagnostics and suppresses duplicate warnings. It augments the short versions with the more explicative and endearing descriptions found in Chapter 33. Like other pragmas, it also affects the compilation phase of your program, not just the run phase.

When you use diagnostics at the start of your program, this automatically enables Perl's -w command-line switch by setting $^W to 1. The remainder of your whole compilation will then be subject to enhanced diagnostics. These still go out on STDERR.

Because of the interaction between run-time and compile-time issues, and because it's probably not a good idea anyway, you may not use no diagnostics to turn them off at compile time. However, you may control their behavior at run time using the disable and enable methods. (Make sure you do the use first, or else you won't be able to get at the methods.)

The -verbose flag first prints out the perldiag manpage's introduction before any other diagnostics are issued. The $diagnostics::PRETTY variable can be set (before the use) to generate nicer escape sequences for pagers like less (1) or more (1):

BEGIN { $diagnostics::PRETTY = 1 } 
use diagnostics;

Warnings dispatched from Perl and detected by this pragma are each displayed only once. This is useful when you're caught in a loop that's generating the same ...