In short because it is all or nothing. It is a global flag and thus enables wanrings in modules you didnt write (which may have been written deliberately without warnings enabled for some reason.) Also because use warnings enables you to selectively enable or disable warnings for various types of problem as well as for the individual modules you are using (assuming that they have been written with warnings::register and its functions). Not only that you can selectively make certain types of warning fatal, enabling you have your code die if certain types occur.

In long heres a copy of the section "Whats wrong with -w and $^W" from perllexwarn

---

What's wrong with -w and $^W

Although very useful, the big problem with using -w on the command line to enable warnings is that it is all or nothing. Take the typical scenario when you are writing a Perl program. Parts of the code you will write yourself, but it's very likely that you will make use of pre-written Perl modules. If you use the -w flag in this case, you end up enabling warnings in pieces of code that you haven't written.

Similarly, using $^W to either disable or enable blocks of code is fundamentally flawed. For a start, say you want to disable warnings in a block of code. You might expect this to be enough to do the trick:

     {
         local ($^W) = 0 ;
         my $a =+ 2 ;
         my $b ; chop $b ;
     }

When this code is run with the -w flag, a warning will be produced for the $a line -- "Reversed += operator".

The problem is that Perl has both compile-time and run-time warnings. To disable compile-time warnings you need to rewrite the code like this:

     {
         BEGIN { $^W = 0 }
         my $a =+ 2 ;
         my $b ; chop $b ;
     }

The other big problem with $^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 -w flag), the second call to doit will trip a "Use of uninitialized value" warning, whereas the first will not.

    sub doit
    {
        my $b ; chop $b ;
    }

    doit() ;

    {
        local ($^W) = 1 ;
        doit()
    }

This is a side-effect of $^W being dynamically scoped.

Lexical warnings get around these limitations by allowing finer control over where warnings can or can't be tripped.

---

Yves / DeMerphq
---
Writing a good benchmark isnt as easy as it might look.