That's not true. use warnings is blocked-scoped or file-scoped if not in a block, while -w will affect modules too.

>type script.pl
use warnings;
use Bla;
Bla->test();

>type bla.pm
package Bla;
sub test { print(undef); }
1;

>perl script.pl

>perl -w script.pl
Use of uninitialized value in print at Bla.pm line 5.
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-w is actually equivalent to BEGIN { $^W = 1 } at the top of your script.

What you should have said is that use warnings is redundant when using -w.