A similar trick can be used to make sure code that uses use warnings works under pre-5.6 as well:

BEGIN {
    if ($] < 5.006) {
        $INC{'warnings.pm'} = __FILE__;
        no strict 'refs';
        *{'warnings::unimport'} = sub {0};
    }
}
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Of course, no warnings will be enabled when run under pre-5.6, but the code will not croak on a use warnings;