my @b = @{ reduce { push @$a, $b if !@$a || $b ne $a->[-1]; $a } [], @a };

##</code><code>##

my @b = list_reduce { push @$_, $b if !@$_ || $b ne $_->[-1]; undef } undef, @a;

my @b = list_reduce { push @$_, $b if $a || $b ne $_->[-1]; 0 } 1, @a;

my @b = list_reduce { push @$_, $b if defined($b) && $b ne $a; $b } undef, @a;

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