Though, this approach also has the disadvantage of silently failing if your "no warnings" should be supported but something weird is just broken. You could mitigate that problem by emitting a warning (or fatal error) if the failure reason doesn't match the one expected, but that can lead to noise when your expectations become incorrect, of course.

BEGIN {
    my $class = 'experimental::signatures';
    if( eval "no warnings '$class'; 1" ) {
        warnings->unimport($class);
    } elsif( $@ !~ /Unknown warnings category / ) {
        warn "'no warnings' failed: $@\n";   # Or even die().
    }
}
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