Writing Safe Code

Using the warnings module or the -w switch

The warnings module is new in Perl version 5.6.0. If you use an older version of Perl, use the -w switch.

Unlike -w, which applies to the entire program, the scope of the warnings module is limited to the enclosing block. Also, it is possible to disable warnings in the middle of a block by the command no warnings;

To read more about warnings, look at the perllexwarn and perldiag manual pages. e.g. on a unix command line type: man perllexwarn.

Example 1a - using the warnings module

#!/usr/local/bin/perl

use warnings;

my $a = 4;
my $b = 5;
my $c;

my $sum = $a + $b + $c;   #we used $c in an operation,
                          #though it was not initialized

print "sum: $sum\n";

# Program output:

# Use of uninitialized value in addition (+) at w1.pl line 9.
# sum: 9

Example 1b - using the 'old' -w switch

#!/usr/local/bin/perl    -w

my $a = 4;
my $b = 5;
my $c;

my $sum = $a + $b + $c;   #we used $c in an operation,
                          #though it was not initialized

print "sum: $sum\n";

# Program output:

# Use of uninitialized value in addition (+) at ./w1.pl line 7.
# sum: 9

Example 2

#!/usr/local/bin/perl

use warnings;

my $a = 5;
my $b = "John";

my $sum = $a + $b;   #we used a string ($b)
                     #inside a numeric operation

print "sum: $sum\n";

# Program output:

# Argument "John" isn't numeric in addition (+) at ./w2.pl line 8.
# sum: 5

Example 3

#!/usr/local/bin/perl

use warnings;

my $number1 = 4;
my $number2 = 5;

$number1 = $number1 * 10;
$namber2 = $number2 * 10;     #spelling mistake

print "numbers: $number1, $number2\n";

# Program output:

# Name "main::namber2" used only once: possible typo at ./w3.pl line 9.
# numbers: 40, 5