our @a; local *a = shift;
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can be safely rewritten as

\local our @a = shift;
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but you'd normally want

\my @a = shift;
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since the only reason package variables were used is because lexical variables couldn't be aliased like that.

That means the code should really become

sub sum {
    \my @a = shift;
    my $total = 0;
    $total += $a[ $_ ] for 0 .. $#a;
    return $total;
}
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