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my @b = list_reduce { undef, !@$_ || $b ne $_->[-1] ? $b : () } undef,
+ @a;

...
my @b = list_reduce { $b, defined($b) && $b ne $a ? $b : () } undef, @
+a;

my @b = list_reduce { $b, grep defined && $_ ne $a, $b } undef, @a;