One can simplify your implementation

sub { ( map { splice @_, 0, 2, $_[0] + ($_[1] // 0) } @_ )[-1] }->(0, 
+@list)
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such that one is left with a single splice and without the extra loop pass that necessitates // 0.

sub { ( splice(@_, 0, 0, 0), ( map $_[0] += $_, @_) )[-1] }->(@list)
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Of course, that's really just the same as

sub { splice(@_, 0, 0, 0); map $_[0] += $_, @_; $_[0] }->(@list)
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At which point, you a can give a better name to the variables you do create (despite claims to the contrary).

sub { my $acc = 0; map $acc += $_, @_; $acc }->(@list)
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Finally, canonise.

sub { my $acc = 0; $acc += $_ for @_; $acc }->(@list)
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