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$ perl -e 'my @arr=(1,2,3); $arr[-1]=4; print join " ", @arr, "\n";'
1 2 4
$

$ perl -e 'my @arr; $arr[-1]=4; print join " ", @arr, "\n";'
Modification of non-creatable array value attempted, subscript -1 at -
+e line 1.
$