Please correct me if I am wrong (sometimes I am) but I think that no warnings; works for the -w switch too. I was asking myself the same question a while ago. Try this:

#!/usr/bin/perl5.10 -w

my %hash1 = {
  test1 => 'test1',
  test2 => 'test2'  
};

no warnings;
my %hash2 = {
  test1 => 'test1',
  test2 => 'test2'  
};

use warnings;
my %hash3 = {
  test1 => 'test1',
  test2 => 'test2'  
};
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I get:

Reference found where even-sized list expected at ./test_warnings.pl l
+ine 3.
Reference found where even-sized list expected at ./test_warnings.pl l
+ine 15.
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Now the question is, what is the difference if any between the -w switch and use warnings;?

Update: Answering myself, on perldoc it says: The warnings pragma is a replacement for the command line flag -w , but the pragma is limited to the enclosing block, while the flag is global.

I very recently encountered a spurious(?) error message (on v5.8.2):

Use of uninitialized value in substitution (s///) at (eval 2) line 21
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Which resulted from the addition of one line like this:

   $hash{$var1}{'string'} = $var2;
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I confirmed that the variables were defined, and that the hash entry was created. I then tried:

   { no warnings 'uninitialized'; 
     # and later:  no warnings;
     $hash{$var1}{'string'} = $var2;
   }
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To no avail. Curiously, the error wasn't occurring at that line (confirmed with print statements); but it disappeared if I commented out that line.

Removing -w got rid of the error.