Oops, adjusted:

my @b = @{ reduce { push @$a, $b if !@$a || $b ne $a->[-1]; $a } [], @
+a };
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If one were to make a list version of reduce, the callback would need access to three variables: The list (say $_), the state (say $a), the current value (say $b). The problem could be solved as follows:

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 } un
+def, @a;

my @b = list_reduce { push @$_, grep defined && $_ ne $a, $b; $b } und
+ef, @a;
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(@a may not start with undef for the last two to work properly.)

Update: Added everything after the first block of code.