If you already know that a certain type of error will not occur, why bother doing extra programming to prevent it?

Encountering undef values in the array was also a case of "a certain type of error will not occur". But it did, due to the one-off error. But supposing there was a valid reason to be seeing undef's in the arrays; you don't want to turn off other warnings that might indicate different errors.

Computers don't just get up in the morning and decide to give you letters instead of numbers as an April Fool's Day joke.

Sure they do; we get it reported here all the time, with code that looks like:

sub foo { while (<INPUT>) { ... } }
for (@array_of_numbers) {
   ...
   foo()
   ...
}
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