Wild guess here but since the anonymous array is on the right side of a list operator, then the comma has really low precedence -- lower than assignment. By adding the +0, you give both terms equal precedence so it goes back to leftward precedence? Well ... that's my guess.

update: what Eily says makes better sense given the simple example of

use Data::Dumper;

my $c = 0;
my $d = [ $c, $c += 2 ];

print Dumper( $d );
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