Learning Perl, Fourth Edition

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.)^[46]

    #!/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 ...