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
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Writing a good benchmark isnt as easy as it might look.

For one, use warnings gives you more control over the warnings you receive. See perldoc warnings for more info.