If the array is empty, then -1 goes to -1 which cannot exist

If the array is not empty, then -1 goes to the last element (it exists)

$ perl -Mdiagnostics -e " my @f; $f[-1] = 0; "
Modification of non-creatable array value attempted, subscript -1 at -
+e line 1 (#1)
    (F) You tried to make an array value spring into existence, and th
+e
    subscript was probably negative, even counting from end of the arr
+ay
    backwards.

Uncaught exception from user code:
        Modification of non-creatable array value attempted, subscript
+ -1 at -e line 1.

$ perl -Mdiagnostics -e " my @f = 0; $f[-1] = 0; "
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