If I understand perllexwarn, which I am not sure that I do, -w does set $^W globally. (And -W makes it irrevocable). Of course, you can't use the shebang in a module.

While in 5.6+ you could put use warnings; in a module and have it scoped to that file only, I think that setting $^W would be global unless enclosed in a block, or unless it was somehow contained.

package foo;

use strict;
$^W = 1;
print "Testing\n", undef; 

1;
__END__

use strict;
use foo;

print "Testing\n", undef;
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In the above example, both print statements throw a warning. However, if I do:

local $^W = 1;
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Then I only get one in the module. Of course, the main thing that perllexwarn seems to say is "don't do any of this stuff." It seems better to take warnings as an all-or-nothing deal in 5.005.

</ajdelore>

I use #!perl -w only in the test suite.
This way, users can decide whether to enable warnings or not.

Update: as I understand from perlrun, a shebang line in a module would not be interpreted at all.