Use the source, Luke! Have you looked at the source code? :-)

my $old_warn_handler = $SIG{__WARN__}; 
$SIG{__WARN__} = sub { 
    if ($_[0] !~ /^Useless use of .+ \(.+\) in void context/) {
        goto &$old_warn_handler if $old_warn_handler;
        warn(@_);
    }
};
[download]

It looks like he basically turned off that warning globally.