Unfortunately it is not set to a value other than zero when the globbing produced no result

No matches is not an error. You could check for that condition explicitely.

if (GLOB_ERROR || !@list) {
   ... error or no match ...
}
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But then, why is it reset with $pat4 in the above example??

The value of $! is meaningless on success. It could be zero, non-zero, undef, etc.