Section 2.3.  Perl's Built-in Warnings

2.3. Perl's Built-in Warnings

Perl can be told to warn you when it sees something suspicious going on in your program. To run your program with warnings turned on, use the -w option on the command line:

    $ perl -w my_program

Or, if you always want warnings, you may request them on the #! line:

    #!/usr/bin/perl -w

That works even on non-Unix systems where it's traditional to write something like this, since the path to Perl doesn't generally matter:

    #!perl -w

With Perl 5.6 and later, you can turn on warnings with a pragma. (Be careful, because it won't work for people with earlier versions of Perl.)^[*]

^[*] The warnings pragma allows lexical warnings, but you'll have to see the perllexwarn manpage to find out about those.

    #!/usr/bin/perl
    use warnings;

Now, Perl will warn you if you use '12fred34' as if it were a number:

    Argument "12fred34" isn't numeric

Of course, warnings are generally meant for programmers and not for end-users. If a programmer doesn't see the warning, it probably won't do any good. And warnings won't change the behavior of your program except that now it will emit gripes once in a while. If you get a warning message you don't understand, you can get a longer description of the problem with the diagnostics pragma. The perldiag manpage has the short warning and the longer diagnostic description.

    #!/usr/bin/perl
    use diagnostics;

When you add the use diagnostics pragma to your program, it may seem to you that your program now pauses for a moment whenever you launch it. That's because your program has to do a lot of work (and gobble a chunk of memory) in case you want to read the documentation as soon as Perl notices your mistakes, if any. This leads to a nifty optimization that can accelerate your program's launch (and memory footprint) with no adverse impact on users, once you no longer need to read the documentation about the warning messages produced by your program, remove the use diagnostics pragma. (It's even better if you fix your program to avoid causing the warnings. But it's sufficient merely to finish reading the output.)

A further optimization can be had by using one of Perl's command-line options, -M, to load the pragma only when needed instead of editing the source code each time to enable and disable diagnostics:

    $ perl -Mdiagnostics ./my_program
    Argument "12fred34" isn't numeric in addition (+) at ./my_program line 17 (#1)
        (W numeric) The indicated string was fed as an argument to
        an operator that expected a numeric value instead.  If you're
        fortunate the message will identify which operator was so unfortunate.

As we run across situations in which Perl will usually be able to warn us about a mistake in your code, we'll point them out. But you shouldn't count on the text or behavior of any warning staying the same in future Perl releases.