First, it's not equivalent since you don't handle the empty list.

my $sum = sub { (map {splice @_, 0, 2, $_[0] + ($_[1] // 0)} @_)[-1] }
+->(@list);
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should be

my $sum = sub { (map {splice @_, 0, 2, $_[0] + ($_[1] // 0)} @_)[-1] }
+->(0, @list);
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A general form of this exists as reduce in List::Util. It's is easier to use (since handling one-element lists is easier) and easier to read (since it uses $a and $b instead of $_[0] and $_[1] and uses less extraneous code).

my $sum = reduce { $a + $b } 0, @list;
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