How do you check for the "specific warning that definitely should not happen?"

I have a simple little function that I include in my test libraries, for example:

sub warns (&) {
    my $sub = shift;
    my @warns;
    { local $SIG{__WARN__} = sub { push @warns, shift };
        $sub->() }
    return wantarray ? @warns : scalar @warns;
}
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Then I'll use it like this:

sub some_module_code {
    my $arg = shift;
    warn "Some custom warning about $arg";
}

my @w = warns {
    some_module_code("foo");
};
is grep({/\bSome custom warning\b/i} @w), 1;
is grep({/\bUse of uninitialized value\b/i} @w), 0;
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