For example, consider the code below:
use warnings ;
- my $a ;
- my $b ;
+ my @a ;
{
no warnings ;
- $b = 2 if $a EQ 3 ;
+ my $b = @a[0] ;
}
- $b = 1 if $a NE 3 ;
+ my $c = @a[0];
The code in the enclosing block has warnings enabled, but the inner
-block has them disabled. In this case that means that the use of the C<EQ>
-operator won't trip a C<"Use of EQ is deprecated"> warning, but the use of
-C<NE> will produce a C<"Use of NE is deprecated"> warning.
+block has them disabled. In this case that means the assignment to the
+scalar C<$c> will trip the C<"Scalar value @a[0] better written as $a[0]">
+warning, but the assignment to the scalar C<$b> will not.
=head2 Default Warnings and Optional Warnings
my $b ; chop $b ;
}
-The other big problem with C<$^W> is that way you can inadvertently
+The other big problem with C<$^W> is the way you can inadvertently
change the warning setting in unexpected places in your code. For example,
when the code below is run (without the B<-w> flag), the second call
to C<doit> will trip a C<"Use of uninitialized value"> warning, whereas
all -+
|
- +- chmod
- |
+- closure
|
+- exiting
|
+- taint
|
- +- umask
- |
+- uninitialized
|
+- unpack
The presence of the word "FATAL" in the category list will escalate any
warnings detected from the categories specified in the lexical scope
-into fatal errors. In the code below, there are 3 places where a
-deprecated warning will be detected, the middle one will produce a
-fatal error.
-
+into fatal errors. In the code below, the use of C<time>, C<length>
+and C<join> can all produce a C<"Useless use of xxx in void context">
+warning.
use warnings ;
- $a = 1 if $a EQ $b ;
+ time ;
{
- use warnings FATAL => qw(deprecated) ;
- $a = 1 if $a EQ $b ;
+ use warnings FATAL => qw(void) ;
+ length "abc" ;
}
- $a = 1 if $a EQ $b ;
+ join "", 1,2,3 ;
+
+ print "done\n" ;
+
+When run it produces this output
+
+ Useless use of time in void context at fatal line 3.
+ Useless use of length in void context at fatal line 7.
+
+The scope where C<length> is used has escalated the C<void> warnings
+category into a fatal error, so the program terminates immediately it
+encounters the warning.
+
=head2 Reporting Warnings from a Module
bless [], $class ;
}
-
+
1 ;
The code below makes use of both modules, but it only enables warnings from