- it is easier for people who are on non 'shebang supporting' systems to use.
- it can be disabled for certain blocks etc. Here's an introduction to the new pragma from the What's new in Perl 5.6 page:

Lexical Warnings

The way Perl generates warnings has also been completely revised: as a replacement for the -w flag and the $^W special variable, the warnings pragma gives you more flexibility about what warnings you receive and when. In terms of what, you can now specify warnings by category: there are a bunch of standard categories, such as 'syntax', 'io', 'void', and modules will be able to define their own categories. You can also choose to escalate any categories of warning into a fatal error. As for when, the pragma is lexically scoped, so you can switch it on and off as you wish:

use warnings;

$a = @a[1];      # This generates a warning.

{
  no warnings;
  $a = @a[1];  # This does not.
}
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As far as I know, the use warnings pragma was introduced as:

- it is easier for people who are on non 'shebang supporting' systems to use.
- it can be disabled for certain sections of code, e.g.

use strict;
use warnings; 

print $foo;   # Warning here, undefined value

no warnings; 
print $foo;   # No warning now 

use warnings; 
print $foo;  # Warning again
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perl -w foo.pl

every time you want warnings. If you can just use the pragma, it is easier.